Widespread phylogenetic conservation of H-Y antigen indicates persistence of a vital function. It has been proposed that this function is the primary determination of mammalian sex. According to this proposal, the indifferent embryonic gonad is induced to differentiate as a testis in the presence of H-Y antigen, and as an ovary in the absence of H-Y antigen. But presence of H-Y antigen does not guarantee testicular differentiation. Other factors may be required: a gene that activates the H-Y structural locus, and another gene that codes for specific H-Y antigen receptors.