Probably plays a regulatory role in sperm development. May participate in regulation of fibrous sheath (FS) assembly by supporting the formation of disulfide bonds during sperm tail morphogenesis. May also be required to rectify incorrect disulfide pairing and generate suitable pairs between the FS constituents. Can reduce disulfide bonds in vitro in the presence of NADP and thioredoxin reductase. In ejaculated spermatozoa, it localizes in the caudal region of the head to the end of the principal piece. Testis-specific. Only expressed during spermiogenesis, prominently in round and elongating spermatids.