A gene deriving from the ancestral sex chromosomes was lost from the X and retained on the Y chromosome in eutherian mammals.

PRSSLY appears to be the first example of a gene that derives from the mammalian ancestral sex chromosomes that was lost from the X and retained on the Y. Although the function of PRSSLY remains to be determined, it may influence the sex ratio by promoting the survival or propagation of Y-bearing sperm.