The sex gene SRY has recently been characterized after years of research during which the candidate genes HY and ZFY were excluded. SRY does have all the characteristics expected of the sex gene: it is located on chromosome Y of all mammals, including man; it is specifically expressed in male gonads during early embryonic development mutations and deletions have been found in 46,XY women with gonadal dysgenesis; the Sry gene is present in the vast majority of XX men and finally, transgenic experiments in the mouse have shown that Sry reverses the sex of XX mice. All this demonstrates that SRY is truly the sex gene, but some abnormalities of sex determination in humans remain shrouded in mystery. There are 46,XX men who do not carry the sex gene, and some 46,XX SRY positive true hermaphrodites in whose gonads ovarian and testicular tissues coexist. Finally, SRY is apparently normal in the majority of pure gonadal dysgenesis.