A genomic sequence (12f3), derived from the long arm of the human Y chromosome, detects a 1·6 kb mRNA, expressed in human and mouse testis, but not in other tissues tested by Northern blot analysis. Using 12f3 as a probe, a mouse cDNA, designated PL5, was isolated from an adult mouse testis cDNA library. The profile obtained by Southern blot analysis using PL5 as probe under high-stringency conditions, reveals that 12f3 probably represents a Y-located pseudogene which was derived from an autosomally located gene. Southern blot analysis of different vertebrate species, using probe PL5, shows that this gene has been highly conserved during evolution. Preliminary in situ hybridizations on testis tissue sections indicate that PL5 is expressed during the postmeiotic stages of male germ cell differentiation and thus may play a role during spermatogenesis. A second cDNA, also obtained from the testis cDNA library, weakly crossreacts with 12f3. This cDNA, designated PL10, detects a mRNA of approximately 4 kb which is highly expressed in mouse testis, but not in male or female mouse liver. The gene corresponding to this cDNA is also well conserved among vertebrates.