May perform a specialized role during sperm development and maturation. According to PubMed:15645076 it is cytoplasmic. Expressed exclusively in testis. Found in spermatagonia, spermatocytes, round spermatids, elongating spermatids and spermatozoa. Expressed only in the adult. Levels are low 1 week postpartum, steadily increase 2 to 5 weeks postpartum, are highest at 7 weeks and then drop to back the levels found at 5 weeks. Belongs to the cystatin family.