Sex reversal syndrome (XX male). Men who appear normal and live a normal life, may have a 46,XX karyotype and present with the typical features of infertility and end organ (testicular) failure. They are azoospermic and their small testicles show specific patterns on light and electron microscopy. Recent advances in genetics (1) favor the "X-Y interchange" theory to explain this phenomenon; (2) hypothesize about the roles of the H-Y antigen and testis determining factor (TDF) in determining "maleness"; and (3) allow mapping of the relative positions of H-Y and TDF loci on the Y chromosome. 