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. Testis-specific. Strongly expressed in the testicular seminiferous tubules, mostly in the round spermatids. Expressed during spermiogenesis, restricted to the postmeiotic phase of spermatogenesis. First detected in elongating spermatids tails during steps 9 and 10 and is prominent in this region during steps 11-16. Also weakly present in the cytoplasmic lobe of these spermatids. During the last steps of spermiogenesis (steps 17-19), it strongly diminishes in the tail but appears to increase or become concentrated in the shrinking cytoplasmic lobe. By the last step of spermiogenesis (late step 19), cytoplasmic localization is barely detectable in the resulting residual body but still detectable in the cytoplasmic droplet (at protein level). Detected in testis of pre-pubertal animals at very low level.