May be a serine protease inhibitor (By similarity). Essential for sperm maturation and fertility. Inhibits sperm acrosome reaction, protecting sperm from premature reaction (By similarity). Secreted into the lumen of the initial segment of the epididymis and binds to sperm. In the initial segment of epididymis, localizes on the dorsal surface of the acrosomal region of sperm, gradually becomes more restricted to the acrosomal region in spermatozoa during epididymal transit (By similarity).