Involved in sperm motility. Its absence is associated with genetic background dependent male infertility. Infertility may be due to reduced sperm motility in the female reproductive tract and inability to penetrate the oocyte zona pellucida. Becomes associated with the spermatid mitochondrion capsule at step 16 of spermatogenesis. Testis. Is selectively expressed in the spermatids of seminiferous tubules. First detected in step 11 spermatids. Levels increase in subsequent steps to reach a maximum in late step 15 and early step 16. Levels decrease in late step 16. Was originally (PubMed:2303168 and PubMed:1418626) thought to be a selenoprotein and was known as sperm mitochondrial capsule selenoprotein.