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 (By similarity). Becomes associated with the spermatid mitochondrion capsule at step 16 of spermatogenesis. Testis. Selectively expressed in the spermatids of seminiferous tubules and in flagella of epididymal sperm. Expressed in postmeiotic cells. Was originally thought to be a selenoprotein and was known as sperm mitochondrial capsule selenoprotein.