May be a serine protease inhibitor (By similarity). Essential for sperm maturation and fertility. Inhibits sperm acrosome reaction, protecting sperm from premature reaction. 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. Restricted to the epididymis, with highest levels in the initial segment, including epithelial cells, lumen, and sperm (at protein level). Localizes to the sperm heads, where it is restricted to the acrosomal region in epididymal spermatozoa, but not in testicular spermatozoa (at protein level). First expressed at low levels at P15 in the epididymis. Expression increases from P30 onward. Reaches its highest level at P120 and remains at a stable level in mature animals. Up-regulated by testosterone.