This protein is supposed to help spermatozoa undergo functional maturation while they move from the testis to the ductus deferens. Stored in secretory granules of granular convoluted tubules cells. Mainly found in the cauda epididymis where it is synthesized by the principal cells and secreted into the lumen. Binds to the heads of spermatozoa. Also expressed in the submandibular gland. Exponential increase between days 25 and 30 after birth. By androgens. Belongs to the CRISP family.