Cdc50p is a transmembrane protein required for polarized growth in Saccharomyces cerevisiae.The knowledge on physiological functions of its mammalian homologs, however, is limited. Using database analysis, we identified one mouse testis expressed sequence tag, named Cdc50c, encoding a previously uncharacterized homolog of Cdc50p. Similar to yeast Cdc50p, the putative Cdc50c protein contains three transmembrane spanning regions. Its orthologs are present in many species such as fish, avian and human,suggesting its evolutionary conservation. In multitissue reverse transcription-polymerase chain reaction analyses the mRNA for Cdc50c was predominately detected in testis. The onset of the gene expression coincides with the first appearance of spermatocytes during testicular development. In situ hybridization analyses revealed that Cdc50c mRNA localized in pachytene spermatocytes and round and elongated spermatids. Our data suggest that Cdc50c might play important roles during spermatogenesis.