Sex differentiation is determined by a cascade of events proceeding from chromosomal sex to the completion of sexual maturation at puberty. Many factors involved in this cascade have been identified. Here we focus on DAX-1, androgen receptor and cytochrome P450c17, and discuss their functions in sex differentiation. We analyzed the DAX-1 genes of two unrelated Japanese patients with congenital adrenal hypoplasia and hypogonadotropic hypogonadism using PCR amplification of genomic DNA and complete exonic sequencing, and established that congenital adrenal hypoplasia and hypogonadotropic hypogonadism result from not only inherited but also de novo mutation in the DAX-1 gene. Androgen insensitivity syndrome (AIS) is a good model to clarify the relationship between the structure and function of androgen receptor, the androgen receptor gene mutation and clinical phenotype. We analyzed 15 cases of AIS and demonstrate the structural and functional relationships of the androgen receptor. We have sequenced the CYP17 (P450c17) gene in DNA from several patients with 17 alpha-hydroxylase deficiency, reconstructed the mutations in a human P450c17 cDNA and expressed the mutant P450c17 in COSl cells to characterize the kinetic properties of 17 alpha-hydroxylase and 17,20-lyase activities. The molecular bases of cases clinically reported as 17 alpha-hydroxylase deficiency have turned out to be complete or partial combined deficiencies of 17 alpha-hydroxylase/17,20-lyase.