The X-linked sex-determining region Y box 3 (SOX3) gene is expressed in the developing gonads and brain. Sox3-null mice developed according to genetic sex, but the hemizygous null males were hypogonadal, with extensive Sertoli cell vacuo-lization, loss of germ cells, and reduced sperm count. We hypothesized that SOX3 mutations might occur in a subset of infertility patients. Genomic DNA samples from 56 infertile men with idiopathic oligo-azoospermia were screened for SOX3 mutations. Three nucleotide substitutions (609 T-->C, 732 A-->C, and 978 G-->A) were identified, none of which altered the amino acid sequence, suggesting that they are polymorphic variants. The 609 T-->C substitution was in the HMG box, and the two other substitutions were identified within the polyalanine repeat regions. Three patients had 609 T-->C, 2 patients had 609 T-->C and 732 A-->C, and one had 978 G-->A. These data indicate that mutations in the SOX3 gene are not a common cause of male infertility.