Does not function as an inhibitor of trypsin, chymotrypsin, subtilisin or elastase. Binds sperm and enhances sperm motility. May act as a decapacitation factor, suppresses BSA-stimulated sperm capacitation and blocks sperm-oocyte interactions in vitro. Luminal fluid and mucosal folds of the seminal vesicles (at protein level). Not detected in brain, heart, lung, liver, kidney, stomach, small intestine, muscle, skin, thymus, placenta or bladder. First appears at low levels in 3 week old mice. Levels increase rapidly after 4 weeks, highest levels are reached in 8 week old mice. By testosterone.