HIGHLIGHTED TOPICS
Genome and Hormones: Gender Differences in Physiology
Invited Review: Sex-based differences in gene expression

Human Genetics Program, Department of Pediatrics, New York University School of Medicine, New York, New York 10016

	ABSTRACT

TOP ABSTRACT INTRODUCTION MOLECULAR GENETICS OF SEX... SEXUALLY DIMORPHIC GENE... CONCLUSION REFERENCES

Certain diseases are more prevalent among women than men. The reasons for this increased prevalence are unknown, but there could be a genetic basis. Increased expression of X-linked genes in females, protective effects of Y-linked genes in males, or sex-limited gene expression that is developmentally or hormonally regulated could all account for these differences. Analysis of individuals with and without genetic sex reversal provides a means for distinguishing between genetic and hormonal causes. This can be complemented by genetic linkage and gene expression profiling to aid in the identification of candidate genes.

Y-linked genes; X-linked genes; sex determination; sex-limited gene expression

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MOLECULAR GENETICS OF SEX... SEXUALLY DIMORPHIC GENE... CONCLUSION REFERENCES

WHEN MENDEL'S LAWS OF HEREDITY were rediscovered, they were applied rapidly to the study of human traits (8, 9, 35, 56). Within a short time, it became apparent that a number of traits, although seemingly genetic, represented deviations from Mendel's rules. The first deviation from Mendelism, sex-linked inheritance, sought to explain why men and women were phenotypically different and why some traits were manifested more commonly in one sex. E. B. Wilson and Nettie Stevens observed differences in the chromosomal constitution of males and females in certain species. This led to the chromosomal theory of sex determination and sex-based differences (63). Sex-linked inheritance was found to be applicable in humans for hemophilia and color blindness and subsequently for other traits (4). With the advent of molecular genetics in the latter 20th century and more recently the sequencing of the whole human genome, many X-linked and Y-linked genes have been identified that can account for some of the phenotypic variability.

Genetic studies of sex-based differences were complemented by work in developmental biology and endocrinology. The pioneer in this field was Alfred Jost, who conducted a series of experiments that provided the framework for how the reproductive apparatus of males develop in response to hormonal signals transmitted by the testes and how the absence of such signals leads to the development of the reproductive tract of females (23). In the 1950s, the fields of genetics and developmental biology coalesced and led to the identification of individuals with genetic sex reversal, that is, people with a phenotype of one sex and a chromosomal constitution of the other. The study of such individuals, whether as sporadic or familial cases, was invaluable for gaining insight into the genetic pathways involved in sex determination (41). The study of such individuals could also prove to be invaluable for understanding how hormones, sex-linked genes, and sex-specific developmental pathways could account for other phenotypic differences between males and females. Here, I review the molecular genetics of sex chromosomes and differences in gene expression between males and females, some of which are known to affect sexual differentiation, with the goal of creating a conceptual framework for how the genetic basis of phenotypic differences between males and females might be approached.

	MOLECULAR GENETICS OF SEX CHROMOSOMES

TOP ABSTRACT INTRODUCTION MOLECULAR GENETICS OF SEX... SEXUALLY DIMORPHIC GENE... CONCLUSION REFERENCES

Several features emerged from the original work with sex-linked transmission (36). Males have an XY chromosomal constitution, whereas females have an XX chromosomal constitution. As a result, the sex of the individual demonstrates a paternal origin effect. X-linked traits do not show a pattern of male-to-male transmission. Rather, in sons, X-linked traits demonstrate a maternal origin effect. To compensate for differences in gene dosage between males and female, a mechanism of X chromosome inactivation developed (see Molecular genetics of the human X chromosome below).

Sex chromosomes differ from autosomes in their organization. The sex chromosomes have two regions, a pseudoautosomal segment shared between X and Y chromosomes and a sex-limited region. The pseudoautosomal regions of the X and Y chromosomes pair at the tips of their short and long arms and undergo recombination during meiosis in spermatocyte precursors (22). The term pseudoautosomal is used because alleles that are inherited within the region are not transmitted exclusively to males or females and thus behave as if they were inherited on autosomes (52). Sex-based differences in gene expression may occur from the sex-limited regions of the X or Y chromosomes. Genes within the sex-limited regions of the X and Y chromosomes are linked to the sexual phenotype of the individual. Genes in the sex-limited region of the Y chromosome have a male-only pattern of transmission.

Molecular genetics of the human Y chromosome. Some of the genes in the sex-limited region of the Y chromosome have functions that could occur only in males, such as testis determination or spermatogenesis (27). Testis determination is mediated by the SRY gene (for "sex-determining region Y") (16, 53). This gene is conserved on the Y chromosomes of most mammals and functions as a developmental switch. Once expressed in the gonadal ridge of humans, it remains on through the period of testicular differentiation and leads the enhanced expression of SF1 and other sex-determining genes in humans (18). Sry has been shown to function as a transcriptional activator in mice (10). Its mechanism of action in humans is unknown, although it has been proposed to function as a repressor of a repressor (33). The SRY gene has been shown to be expressed in early male embryos and in the central nervous system of humans and mice (32). This observation has led to the suggestion that expression of SRY could account for hormone-independent, somatic differences between males and females.

Genes that promote spermatogenesis have been shown to exist in three different regions of the Y chromosome because deletion of one or more of these regions is found in individuals with oligo/azospermia (20, 25, 31, 60). Deletions of these regions remove one or more of the candidate genes (DAZ, RBMY, USP9Y, and DBY). Of these, DAZ is a favored candidate because DAZ protein appears to be expressed in the right place at the right time. This protein is present in both the nuclei and cytoplasm of fetal gonocytes and in spermatogonial nuclei; during male meiosis, it relocates to the cytoplasm (46).

A number of other genes have been identified on the Y chromosome. Some of these are present as single genes on the Y, whereas others have multiple copies on the Y or homologous copies on the X and Y chromosomes (54). With the use of deletion mapping techniques, functional significance has been ascribed to some of these genes, including those for stature, suppression of gonadoblastoma, and prevention of the Turner syndrome phenotype (39, 40, 57). Differences in histocompatibility can also be ascribed to Y-linked genes. The SMCY gene encodes the HY antigen, which is ubiquitously expressed in male tissues, as early as two-cell embryos (1). HY is a minor histocompatibility antigen that can promote immunologically mediated rejection of male tissues transplanted into female mice (14). SMCY has an X-linked ortholog from which it differs by the presence of an HY epitope (defined by the octamer peptide TENSGKDI) that presumably accounts for the antigenic difference between males and females (51). The UTY gene also encodes a male-specific histocompatibility antigen that is recognized by female T cells in a major histocompatibility complex-restricted manner (61).

Other genes with X-Y homologs may serve similar functions but may differ from each other in subtle ways. The RPS4Y and RPS4X genes encode ribosomal binding proteins that differ at 19 of 263 amino acids. Both genes are widely transcribed in human tissues, which suggests that the ribosomes of men and women are structurally, if not functionally, distinct (12).

Molecular genetics of the human X chromosome. X-linked traits are characterized by absence of male-to-male transmission; they may be transmitted from fathers to daughters or from mothers to daughters and sons. If one copy of the mutant gene is required for expression in females, then the trait is dominant. If two copies are required, then the trait is recessive. The presence of the hemizygous state in males makes it easier to map the genes for X-linked recessive conditions than the genes for autosomal disorders. To date, over 1,200 X-linked conditions have been described (34). Some conditions are distinctive in their phenotypic features, whereas others, such as mental retardation, are nonspecific (35). The pattern of inheritance can infer the X-linked nature of these conditions. Precise mapping to specific regions of the X chromosome can be performed by linkage to specific markers. X-linked dominant conditions, such as incontinentia pigmenti, Rett syndrome, microphthalmia with linear skin defects, and Aicardi syndrome, may not show Mendelian transmission because they are expressed only in females and usually as sporadic cases. The paucity of male cases for these conditions is presumed to result from lethality during gestation (2, 17, 28, 29, 42).

Most X-linked genes have dosage compensation between males and females by X chromosome inactivation, a process also known as "lyonization." This process of random inactivation of one of the two X chromosomes starts in early embryos (30). The inactivation originates at a site on the long arm of the X chromosome (termed the "X chromosome inactivation center") and spreads over the chromosome (49). The inactivation is stable and is transmitted to the progeny cells. X chromosome inactivation skips over some regions of the X chromosome affecting some genes but not others; thus the degree of inactivation at a given locus is variable, ranging from complete to none at all (50). Some of the loci that escape X chromosome inactivation have homologous genes in either the pseudoautosomal or sex-limited regions of the Y chromosome (27).

Skewing may occur in which one X chromosome may be preferentially inactivated in a majority of cells. This skewing of X chromosome inactivation may be a heritable trait, may occur by chance alone, or may be the result of selection (7, 37, 43, 55). The stochastic nature of X chromosome inactivation has been highlighted by differences in the patterns of X chromosome inactivation among the tissues of an individual and by discordance in the patterns of X chromosome inactivation between monozygotic twins (13, 47). When this occurs for an X chromosome that contains a mutant gene, only one twin in the pair may have a population of cells in which inactivation of the X chromosome bearing a mutant gene predominated and thus is affected with the condition. This appears as discordance of the X-linked phenotype between the twins. Variability in X chromosome inactivation may likewise account for discordance in X-linked phenotypes among singleton sisters who harbor the same mutant allele.

On the other hand, heritable, skewed X chromosome inactivation may account for concordance of X-linked phenotypes among female relatives. Skewed inactivation in some cell types occurs because the expression of a mutant allele (or failure to express the normal allele) prevents cells from maturing along a developmental pathway. The B cells of females heterozygous for the X-linked agammaglobulinemia express only the X chromosome that encodes the normal allele (3, 7). Those precursor cells in which the chromosome with the normal gene is inactivated fail to mature along the B cell pathway. Individuals with balanced X-autosomal translocations tend to have selective inactivation of the normal (or untranslocated) X chromosome (64). Inactivation of the translocated X chromosome is lethal at a cellular level because the inactivation spreads to the autosomal segment and monosomy for the autosomal regions is selected against by the suboptimal dosage of autosomal genes. Skewed inactivation may also arise from mutations in the minimal promoter of the XIST gene; normally, X chromosome inactivation correlates with expression of the XIST gene on the chromosome being inactivated. Transmission of this mutant causes extreme skewing of X chromosome inactivation to be a heritable trait (43).

	SEXUALLY DIMORPHIC GENE EXPRESSION FROM AUTOSOMES

TOP ABSTRACT INTRODUCTION MOLECULAR GENETICS OF SEX... SEXUALLY DIMORPHIC GENE... CONCLUSION REFERENCES

In addition to SRY, the autosomal genes WT1, SOX9, SF1, WNT4, DMRT1 and FGF9 play a role in gonadal development. Temporal and spatial differences have been observed for all of these genes in developing human testes and ovaries, thereby demonstrating a dimorphic genetic pathway for gonadal determination and development (4, 18, 19, 44, 59). Several bits of evidence suggest that the effects of SRY in causing male sexual differentiation may channel through SOX9. The expression of SOX9 in the gonads of 46,XY human embryos follows a pattern similar to that of SRY (18). The expression commences with testicular induction and increases over the next several days with maximal detection observed over the sex cords, most likely in Sertoli cells. A 46,XX male patient was observed with a chromosomal duplication encompassing the SOX9 gene. This finding suggested that enhanced expression of SOX9, even in the absence of SRY, is sufficient to cause testis determination (21). In XX Odsex mice, derepression of Sox9 expression in XX gonads leads to male development (5). Ordinarily, this derepression might be mediated by Sry.

Dimorphic gene expression may occur for other developmental pathways that could influence disease susceptibility. In addition to control from a regulatory sex chromosome-linked gene, such as SRY, genes under the control of sex steroids demonstrate dimorphic gene expression because of differences in the production of these hormones by men and women (48). Such differences have been observed for CYP11A1 in the brain, steroid sulfatase in trophoblasts, and estrogen receptor in the liver, among others. CYP11A1, the gene for the first enzyme in steroid biosynthesis (P450SCC, cholesterol side-chain cleavage enzyme), is expressed in the temporal and frontal lobe cortex of the brain and is significantly higher in women than in men (62). Steroid sulfatase is an enzyme that catalyzes the hydrolysis of the sulfate ester bonds of sulfated steroids, such as cholesterol, dehydroepiandrosterone, and estrone sulfate. Human cytotrophoblasts in primary culture show a gender-specific regulation of steroid sulfatase activity, with an increase of twofold to threefold in female, but not in male, cells (58). The estrogen receptor is a complex genomic unit in mice that exhibits alternative splicing that is regulated in a tissue-specific manner. Five variants have been described that are generated by alternative splicing that differ in their 5' untranslated regions. All encode a 66-kDa estrogen receptor protein, which the previously identified mRNA C variant generates. The expression of the H isoform mRNA is restricted to liver, although female mice produce around a fivefold higher level of this transcript than male mice (26). Most likely, many other sex-specific differences will be found.

Many different factors have been described that affect genes whose expression is influenced by steroid hormones (reviewed in Ref. 45). Some of these factors operate in trans, whereas others operate in cis. The trans elements include the availability of the hormones, their receptors, the receptor transcriptional coregulatory proteins, and basal transcription factors in the responsive tissue. The cis elements are the binding sites for the hormone receptors. For the estrogen receptor, specificity is conferred by the sequence of the DNA response element (24). By contrast, for the androgen receptor, specificity is conveyed by binding cooperatively to multiple androgen responsive elements in native promoters (15).

	CONCLUSION

TOP ABSTRACT INTRODUCTION MOLECULAR GENETICS OF SEX... SEXUALLY DIMORPHIC GENE... CONCLUSION REFERENCES

Studies to date have highlighted how differences in the genetic constitution of individuals, including sex chromosomes, sex-specific expression of genes, and hormonal concentrations, can influence not only their sexual phenotype but also other physical phenotypes, including susceptibility to disease. In addition, the genetic makeup of individuals (whether men or women) can influence the expression of certain autosomal genes in the next generation, a phenomenon that is known as "genomic imprinting."

The process of teasing out sex-based differences in phenotypes should become more efficient. New technology for assaying for quantitative differences in the expression of thousands of genes simultaneously has emerged. These techniques, which commonly fall under the rubric of chip technology, could be applied to different classes of lymphocytes or different target tissues between affected and unaffected males and females (11). For such studies to be meaningful, a rigorous case-control design should be applied. To sort out between primary genetic and hormonal effects, individuals with genetic sex reversal who have the expected genetic constitution, but not the hormonal constitution, for the group most commonly affected might be included in these studies.

Candidate genes identified from these approaches can then be rigorously tested for their roles in disease susceptibility or disease protection using linkage or association. The identification of a new genes should meet several criteria to confirm the role of those mechanism in the development of a specific phenotype. First, the gene should demonstrate linkage or a nonrandom association with a given phenotype that can be rigorously tested using a statistical method, such as LOD scores (for linkage) or X² analysis (for association). Second, the strength of the association should increase as the markers analyzed approach the gene(s) that causes the condition. Finally, there should be a biological assay that demonstrates that the expression of a causative gene has been altered as the result of a mutation that is thought to influence disease susceptibility.

	FOOTNOTES

Address for reprint requests and other correspondence: H. Ostrer, Human Genetics Program, Dept. of Pediatrics, NYU School of Medicine, 550 First Ave., MSB 136, New York, NY 10016 (E-mail: ostreh01{at}med.nyu.edu).

	REFERENCES

TOP ABSTRACT INTRODUCTION MOLECULAR GENETICS OF SEX... SEXUALLY DIMORPHIC GENE... CONCLUSION REFERENCES

1.   Agulnik, AI, Mitchell MJ, Lerner JL, Woods DR, and Bishop CE. A mouse Y chromosome gene encoded by a region essential for spermatogenesis and expression of male-specific minor histocompatibility antigens. Hum Mol Genet 3: 873-878, 1994[Abstract/Free Full Text].

2.   Aicardi, J, Chevrie JJ, and Rousselie F. Le syndrome spasnes en flexion, agenesic calleluse, anomalies chorio-retiniennes. Arch Franc Pediat 26: 1103-1120, 1969[Web of Science][Medline].

3.   Allen, RC, Nachtman RG, Rosenblatt HM, and Belmont JW. Application of carrier testing to genetic counseling for X-linked agammaglobulinemia. Am J Hum Genet 54: 25-35, 1994[Web of Science][Medline].

4.   Bell, J, and Haldane JBS The linkage between the genes for color-blindness and haemophilia in men. Proc R Soc Lond B Biol Sci 123: 119-150, 1937[Free Full Text].

5.   Bishop, CE, Whitworth DJ, Qin Y, Agoulnik AI, Agoulnik IU, Harrison WR, Behringer RR, and Overbeek PA. A transgenic insertion upstream of Sox9 is associated with dominant XX sex reversal in the mouse. Nat Genet 26: 490-494, 2000[Web of Science][Medline].

6.   Colvin, JS, Green RP, Schmahl J, Capel B, and Ornitz DM. Male-to-female sex reversal in mice lacking fibroblast growth factor 9. Cell 104: 875-889, 2001[Web of Science][Medline].

7.   Conley, ME, Brown P, Pickard AR, Buckley RH, Miller DS, Raskind WH, and Singer JW. Expression of the gene defect in X-linked agammaglobulinemia. N Engl J Med 315: 564-567, 1986[Web of Science][Medline].

8.   Correns, C. G. Mendel's Regel über das Verhalten der Nachkommenschaft der Rassenbastarde. Ber Dtsch Bot Ges 18: 158-168, 1900.

9.   De Vries, H. Sur la loi de disjonction des hybrides. C R Acad Sci Paris 130: 845-847, 1900.

10.   Dubin, RA, and Ostrer H. Sry is a transcriptional activator. Mol Endocrinol 8: 1182-1192, 1994[Abstract/Free Full Text].

11.   Duggan, DJ, Bittner M, Chen Y, Meltzer P, and Trent JM. Expression profiling using cDNA microarrays. Nat Genet 21: 10-14, 1999[Web of Science][Medline].

12.   Fisher, EM, Beer-Romero P, Brown LG, Ridley A, McNeil JA, Lawrence JB, Willard HF, Bieber FR, and Page DC. Homologous ribosomal protein genes on the human X and Y chromosomes: escape from X inactivation and possible implications for Turner syndrome. Cell 63: 1205-1218, 1990[Web of Science][Medline].

13.   Gale, RE, Wheadon H, Boulos P, and Linch DC. Tissue specificity of X-chromosome inactivation patterns. Blood 83: 2899-2905, 1994[Abstract/Free Full Text].

14.   Gasser, DL, and Silvers WK. Genetics and immunology of sex-linked antigens. Adv Immunol 15: 215-247, 1972[Medline].

15.   Grad, JM, Dai JL, Wu S, and Burnstein KL. Multiple androgen response elements and a Myc consensus site in the androgen receptor (AR) coding region are involved in androgen-mediated up-regulation of AR messenger RNA. Mol Endocrinol 13: 1896-1911, 1999[Abstract/Free Full Text].

16.   Gubbay, J, Collignon J, Koopman P, Capel B, Economou A, Munsterberg A, Vivian N, Goodfellow P, and Lovell-Badge R. A gene mapping to the sex-determining region of the mouse Y chromosome is a member of a novel family of embryonically expressed genes. Nature 346: 245-250, 1990[Medline].

17.   Hagberg, B, Aicardi J, Dias K, and Ramos O. A progressive syndrome of autism, dementia, ataxia, and loss of purposeful hand use in girls. Rett's syndrome: report of 35 cases. Ann Neurol 14: 471-479, 1983[Web of Science][Medline].

18.   Hanley, NA, Ball SG, Clement-Jones M, Hagan DM, Strachan T, Lindsay S, Robson S, Ostrer H, Parker KL, and Wilson DI. Expression of steroidogenic factor 1 and Wilms' tumour 1 during early human gonadal development and sex determination. Mech Dev 87: 175-180, 1999[Web of Science][Medline].

19.   Hanley, NA, Hagan DM, Clement-Jones M, Ball SG, Strachan T, Salas-Cortes L, McElreavey K, Lindsay S, Robson S, Bullen P, Ostrer H, and Wilson DI. SRY, SOX9, and DAX1 expression patterns during human sex determination and gonadal development. Mech Dev 91: 403-407, 2000[Web of Science][Medline].

20.   Henegariu, O, Hirschmann P, Kilian P, Kirsch S, Lengauer C, Maiwald R, Mielke K, and Vogt P. Rapid screening of the Y chromosome in idiopathic sterile men, diagnostic for deletion in AZF, a genetic Y-factor expressed during spermatogenesis. Andrologia 26: 97-106, 1994[Web of Science][Medline].

21.   Huang, B, Wang S, Ning Y, Lamb AN, and Bartley J. Autosomal sex reversal caused by duplication of SOX9. Am J Med Genet 87: 349-353, 1999[Web of Science][Medline].

22.   Hulten, M, Lindsten J, Ming PM, and Fracaro M. The XY bivalent in human male meiosis. Ann Hum Genet 30: 119-123, 1966[Web of Science][Medline].

23.   Jost, A, Vigier B, Prepin J, and Perchellet JP. Studies of sex determination in mammals. Recent Prog Horm Res 29: 1-41, 1973.

24.   Katzenellenbogen, BS, Choi I, Delage-Mourroux R, Ediger TR, Martini PG, Montano M, Sun J, Weis K, and Katzenellenbogen JA. Molecular mechanisms of estrogen action: selective ligands and receptor pharmacology. J Steroid Biochem Mol Biol 74: 279-285, 2000[Web of Science][Medline].

25.   Kobayashi, K, Mizuno K, Hida A, Komaki R, Tomita K, Matsushita I, Namiki M, Iwamoto T, Tamura S, Minowada S, Nakahori Y, and Nakagome Y. PCR analysis of the Y chromosome long arm in azospermic patients: evidence for a second locus required for spermatogenesis. Hum Mol Genet 3: 1965-1967, 1994[Abstract/Free Full Text].

26.   Kos, M, O'Brien S, Flouriot G, and Gannon F. Tissue-specific expression of multiple mRNA variants of the mouse estrogen receptor alpha gene. FEBS Lett 477: 15-20, 2000[Web of Science][Medline].

27.   Lahn, BT, and Page DC. Functional coherence of the human Y chromosome. Science 278: 675-680, 1997[Abstract/Free Full Text].

28.   Lenz, W. Zur Genetik der Incontinentia pigmenti. Ann Paediat 196: 149-165, 1961.

29.   Lindsay, EA, Grillo A, Ferrero GB, Roth EJ, Magenis E, Grompe M, Hulten M, Gould C, Baldini A, and Zoghbi HY. Microphthalmia with linear skin defects (MLS) syndrome: clinical, cytogenetic, and molecular characterization. Am J Med Genet 49: 229-234, 1994[Web of Science][Medline].

30.   Lyon, MF. Gene action in the X-chromosome of the mouse (Mus musculus). Nature 190: 372-373, 1961[Medline].

31.   Ma, K, Sharkey A, Kirsch S, Vogt P, Keil R, Hargreave T, McBeath S, and Chandley A. Towards the molecular localisation of the AZF locus: mapping of microdeletions in azoospermic men within 14 subintervals of interval 6 of the human Y chromosome. Hum Mol Genet 1: 29-33, 1992[Abstract/Free Full Text].

32.   Mayer, A, Mosler G, Just W, Pilgrim C, and Reisert I. Developmental profile of Sry transcripts in mouse brain. Neurogenetics 3: 25-30, 2000[Web of Science][Medline].

33.   McElreavey, K, Vilain E, Abbas N, Herskowitz I, and Fellous M. A regulatory cascade hypothesis for mammalian sex determination: SRY represses a negative regulator of male development. Proc Natl Acad Sci USA 90: 3368-3372, 1993[Abstract/Free Full Text].

34.   McKusick, VA. Mendelian Inheritance in Man. Baltimore, MD: Johns Hopkins Univ. Press, 1998.

35.   Mendel, G. Versuche über Pflanzen-Hybriden. Verh Naturforsch Ver Brünn 4: 3-47, 1865.

36.   Morgan, TH, Sturtevant AH, Muller HJ, and Bridges CB. The Mechanism of Mendelian Inheritance. New York: Holt, 1915.

37.   Naumova, AK, Plenge RM, Bird LM, Leppert M, Morgan K, Willard HF, and Sapienza C. Heritability of X chromosome-inactivation phenotype in a large family. Am J Hum Genet 58: 1111-1119, 1996[Web of Science][Medline].

38.   Neri, G, Chiurazzi P, Arena F, Lubs HA, and Glass IA. XLMR genes: update 1992. Am J Med Genet 43: 373-382, 1992[Web of Science][Medline].

39.   Ogata, T, Tomita K, Hida A, Matsuo N, Nakahori Y, and Nakagome Y. Chromosomal localisation of a Y specific growth gene(s). J Med Genet 32: 572-575, 1995[Abstract/Free Full Text].

40.   Ogata, T, Tyler-Smith C, Purvis-Smith S, and Turner G. Chromosomal localization of a gene(s) for Turner stigmata on Yp. J Med Genet 30: 918-922, 1993[Abstract/Free Full Text].

41.   Ostrer, H. Sexual differentiation. Semin Reprod Med 18: 41-49, 2000[Web of Science][Medline].

42.   Pfeiffer, RA. Zur Frage der Vererbung der Incontinentia pigmenti Bloch-Siemens. Z Menschl Vererbungs Konstitutionsl 35: 469-493, 1960.

43.   Plenge, RM, Hendrich BD, Schwartz C, Arena JF, Naumova A, Sapienza C, Winter RM, and Willard HF. A promoter mutation in the XIST gene in two unrelated families with skewed X-chromosome inactivation. Nat Genet 17: 353-356, 1997[Web of Science][Medline].

44.   Raymond, CS, Kettlewell JR, Hirsch B, Bardwell VJ, and Zarkower D. Expression of Dmrt1 in the genital ridge of mouse and chicken embryos suggests a role in vertebrate sexual development. Dev Biol 215: 208-220, 1999[Web of Science][Medline].

45.   Reid, KJ, Hendy SC, Saito J, Sorensen P, and Nelson CC. Two classes of androgen receptor elements mediate cooperativity through allosteric interactions. J Biol Chem 276: 2943-2952, 2001[Abstract/Free Full Text].

46.   Reijo, RA, Dorfman DM, Slee R, Renshaw AA, Loughlin KR, Cooke H, and Page DC. DAZ family proteins exist throughout male germ cell development and transit from nucleus to cytoplasm at meiosis in humans and mice. Biol Reprod 63: 1490-1496, 2000[Abstract/Free Full Text].

47.   Richards, CS, Watkins SC, Hoffman EP, Schneider NR, Milsark IW, Katz KS, Cook JD, Kunkel LM, and Cortada JM. Skewed X inactivation in a female MZ twin results in Duchenne muscular dystrophy. Am J Hum Genet 46: 672-681, 1990[Web of Science][Medline].

48.   Roy, AK, and Chatterjee B. Androgen action. Crit Rev Eukaryot Gene Expr 5: 157-176, 1995[Web of Science][Medline].

49.   Russell, LB. Mammalian X-chromosome action: inactivation limited in spread and in region of origin. Science 133: 1795-803, 1963.

50.   Schneider-Gadicke, A, Beer-Romero P, Brown LG, Nussbaum R, and Page DC. ZFX has a gene structure similar to ZFY, the putative human sex determinant, and escapes X inactivation. Cell 57: 1247-1258, 1989[Web of Science][Medline].

51.   Scott, DM, Ehrmann IE, Ellis PS, Bishop CE, Agulnik AI, Simpson E, and Mitchell MJ. Identification of a mouse male-specific transplantation antigen, H-Y. Nature 376: 695-698, 1995[Medline].

52.   Simmler, M-C, Rouyer F, Vergnaud G, Nystrom-Lahti M, Ngo KY, de la Chapelle A, and Weissenbach J. Pseudoautosomal DNA sequences in the pairing region of the human sex chromosomes. Nature 317: 692-697, 1985[Medline].

53.   Sinclair, AH, Berta P, Palmer MS, Hawkins JR, Griffiths BL, Smith MJ, Foster JW, Frischauf AM, Lovell-Badge R, and Goodfellow PN. A gene from the human sex-determining region encodes a protein with homology to a conserved DNA-binding motif. Nature 346: 240-244, 1990[Medline].

54.   Tilford, CA, Kuroda-Kawaguchi T, Skaletsky H, Rozen S, Brown LG, Rosenberg M, McPherson JD, Wylie K, Sekhon M, Kucaba TA, Waterston RH, and Page DC. A physical map of the human Y chromosome. Nature 409: 943-945, 2001[Medline].

55.   Trejo, V, Derom C, Vlietinck R, Ollier W, Silman A, Ebers G, Derom R, and Gregersen PK. X chromosome inactivation patterns correlate with fetal-placental anatomy in monozygotic twin pairs: implications for immune relatedness and concordance for autoimmunity. Mol Med 1: 62-70, 1994[Web of Science][Medline].

56.   Tschermak, E. Uber kunstliche Kreuzung bei Pisum sativum. Z Landw Versuchsw Osterr 3: 5, 1900.

57.   Tsuchiya, K, Reijo R, Page DC, and Disteche CM. Gonadoblastoma: molecular definition of the susceptibility region on the Y chromosome. Am J Hum Genet 57: 1400-1407, 1995[Web of Science][Medline].

58.   Ugele, B, and Regemann K. Differential increase of steroid sulfatase activity in XX and XY trophoblast cells from human term placenta with syncytia formation in vitro. Cytogenet Cell Genet 90: 40-46, 2000[Web of Science][Medline].

59.   Vainio, S, Heikkila M, Kispert A, Chin N, and McMahon AP. Female development in mammals is regulated by Wnt-4 signalling. Nature 397: 405-409, 1999[Medline].

60.   Vogt, P, Chandley AC, Hargreave TB, Keil R, Ma K, and Sharkey A. Microdeletions in interval 6 of the Y chromosome of males with idiopathic sterility point to disruption of AZF, a human spermatogenesis gene. Hum Genet 89: 491-496, 1992[Web of Science][Medline].

61.   Warren, EH, Gavin MA, Simpson E, Chandler P, Page DC, Disteche C, Stankey KA, Greenberg PD, and Riddell SR. The human UTY gene encodes a novel HLA-B8-restricted H-Y antigen. J Immunol 164: 2807-2814, 2000[Abstract/Free Full Text].

62.   Watzka, M, Bidlingmaier F, Schramm J, Klingmuller D, and Stoffel-Wagner B. Sex- and age-specific differences in human brain CYP11A1 mRNA expression. J Neuroendocrinol 11: 901-905, 1999[Web of Science][Medline].

63.   Wilson, EB. The sex chromosomes. Arch Mikrosk Anat Enwicklungsmech 77: 249-271, 1911.

64.   Zabel, BU, Baumann WA, Pirntke W, and Gerhard-Ratschow K. X-inactivation pattern in three cases of X/autosome translocation. Am J Med Genet 1: 309-317, 1978[Web of Science][Medline].