TSPY genes must have been selectively maintained on the mammalian Y chromosome since before the radiation of eutheria, 80 million years ago, as they are found conserved on the Y chromosome in 2 mammalian orders: primate and artiodactyl. The gene structure and expression of rat TSPY suggested that it is a functional, testis-specific gene, but the closely related mouse gene, Tspy, has clearly become nonfunctional, producing only low levels of aberrantly spliced transcripts. Thus, TSPY lost its function in the mouse lineage after its divergence from the rat lineage. These observations appeared to support the impression that recombination prevents genetic deterioration in sexual populations because it allows conservation of functional genotypes by removing deleterious mutations.