A novel cDNA-encoding polypeptide of 545 amino acid residues was identified from a mouse testis cDNA library. The transcript of this gene, p59(scr), was predominantly expressed in the testis and was developmentally regulated during spermatogenesis. The encoded protein was expressed in spermatocytes and round spermatids within seminiferous tubules of the adult testis. Using an adult-mouse model of experimental unilateral cryptorchidism, it was observed that the expression of the p59(scr) mRNA was reduced in the cryptorchid testes in association with destruction of spermatogenesis. In vitro heat stress experiment further demonstrated that p59(scr) mRNA was more sensitive to heat stress than the other mRNA species, such as germ-cell-specific meiosis-activating kinase (mak) gene and a housekeeping beta-actin gene. Our results suggest that this novel p59(scr) gene is involved in spermatogenesis and may play an important role in development of testicular germ cells.