Maspin (mammary serine protease inhibitor) also known as serpin B5 is a protein that in humans is encoded by the SERPINB5 gene.[1] This protein belongs to the serpin (serine protease inhibitor) superfamily.[1] SERPINB5 also functions as a tumor suppressor gene in epithelial cells. The expressed protein suppresses the ability of cancer cells to invade and metastasize other tissues. The protein also functions as an angiogenesis inhibitor.It blocks the growth, invasion, and metastatic properties of mammary tumors. As it does not undergo the S (stressed) to R (relaxed) conformational transition characteristic of active serpins, it exhibits no serine protease inhibitory activity.