Mammary serine protease inhibitor (Maspin), also known as serpin B5 is a non-inhibitory serpin that is encoded by the SERPINB5 gene in humans. The protein is expressed predominantly in normal mammary epithelial cells but at significantly reduced levels or absent in most breast carcinomas. As it does not undergo the S (stressed) to R (relaxed) conformational transition characteristic of active serpins, it exhibits no serine protease inhibitory activity. The maspin functions as tumor suppressor, blocking the growth, invasion, and metastatic properties of mammary tumors. This anti-tumor activity is achieved, in part, by the ability of maspin to inhibit angiogenesis and to preferentially promote apoptosis of tumor cells.