Induction of testis development in mammals requires the presence of the Y-chromosome gene SRY. This gene must exert its effect by interacting with other genes in the sex-determination pathway. Cloning of a translocation chromosome breakpoint from a sex-reversed patient with campomelic dysplasia, followed by mutation analysis of an adjacent gene, indicates that SOX9, an SRY-related gene, is involved in both bone formation and control of testis development.

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Linkage of the chicken major histocompatibility complex (B-complex) to the nucleolus organizer regions (NOR) has been confirmed in situ. Nonradioactive hybridization to metaphase chromosomes using a pool of biotinylated cDNA probes for the B-F, B-L beta, and B-G genes of the B-complex gave positive signals on one or a pair of microchromosomes. Subsequent staining with silver nitrate proved these microchromosomes to be the NOR-bearing pair. The value of employing nonisotopic in situ hybridization techniques for mapping of genes and gene complexes, especially when dealing with microchromosomes, is stressed.

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In eutherian mammals, the Y-chromosome gene SRY is required for induction of testis development. Although the Y chromosome is sex determining, loci located elsewhere in the genome participate in the complex cascade of genetic interactions required to form a testis. Male to female sex reversal (46,XY females) occurs at a high frequency in individuals afflicted with the skeletal malformation syndrome campomelic dysplasia. Chromosomal translocations in individuals with both syndromes had localized an autosomal sex reversal locus (SRA1) and a campomelic dysplasia locus (CMPD1) to the long arm of human chromosome 17. The molecular cloning of a translocation breakpoint in a sex reversed campomelic dysplasia patient revealed its proximity to SOX9, a gene which is related to SRY. Analysis of SO X9 in patients without chromosomal rearrangements demonstrated single allele mutations in sex reversed campomelic individuals, linking this gene with both bone formation and control of testis development. Identification of SO X9 as SRA1/CMPD1 and the role of SO X9 mutations in sex reversal and campomelic dysplasia are discussed.