The mammalian Y chromosome has been known for a long time to harbour a gene that triggers testis determination, and this testis-determining factor was identified as SRY in 1990. It has been supposed that SRY was the original mammalian sex-determining gene that initiated the differentiation of the Y from the X early in mammalian evolution, and this belief has been reinforced by an analysis of divergence times. However, I will argue here that SRY evolved quite recently in therian mammals and was not the original mammalian sex-determining gene that defined the X and Y. It arose as a degraded version of the X-borne SOX gene that is better qualified to be a brain-determining gene. It has no central role in sex determination, and can be replaced as a trigger and lost, as have many other Y-borne genes in recent evolutionary history. The mole vole has evidently accomplished this.