To identify new genes that could be involved in the differentiation and function of male germ cells, we have screened a murine testis cDNA library and isolated a clone that was named Tep22. The gene encoding Tep22 consists of three exons and is localized in the telomeric region of mouse chromosome 12. Expression analyses with RNA from different adult tissues revealed that Tep22 is predominantly expressed in spermatocytes and spermatids of the murine testis. Four Tep22 transcript sizes ranging from 647 to 1122 nucleotides were detected in testes of 15-day-old mice due to variable 5' UTRs, while the open reading frame of Tep22 has a length of 567bp in all transcript forms. Specific antibodies against Tep22 detected an approximately 22kDa band in testicular protein extracts, which was first observed in 18-day-old mice, indicating that Tep22 is translationally repressed for several days. Indirect immunofluorescence and immunoelectron microscopy experiments demonstrate that Tep22 is localized in the acrosomal region of early elongating spermatids, while the surrounding cytoplasm is barely labeled. During further germ cell development, the intensity of the staining in the acrosomal region decreases and is no longer detectable in late stages of elongating spermatids, whereas the amount of the Tep22 protein increases in the cytoplasm. Finally, Tep22 is incorporated into the midpiece of spermatids and is also present in the mitochondrial sheath of mature spermatozoa. Taken together, our results suggest that Tep22 is involved in the biogenesis of the acrosome as well as in the function of the midpiece of murine spermatozoa.