Serine protease inhibitor (Probable) (PubMed:22125495). Probably by inhibiting serine protease tyr-5 in males, prevents the maturation of spermatids into mature motile spermatozoa until their transfer into a hermaphrodite (PubMed:16461278, PubMed:22125495, PubMed:30470702). Also required for efficient sperm transfer and thus for male fertility (PubMed:16461278). In males, partially colocalizes with tyr-5 in vesicles near the apical membrane of cuboidal cells. Secreted predominantly by muscles into the pseudocoelom where it enters the seminal vesicle in males and the spermatheca in hermaphrodites. Localizes around the spermatocytes at the late stages of meiosis. During mating, transferred together with sperm into hermaphrodites where it spread into the uterus. Also, is uptaken by coelomocytes. In male, expressed in the vas deferens cuboidal cells and, in posterior body wall and male-specific diagonal muscles (PubMed:30470702). In hermaphrodites, expressed in posterior body wall muscles and spermatheca (PubMed:30470702). In males, expression begins at late larval stage and continues in adults.