Mammary serine protease inhibitor (Maspin) is a non-inhibitory tumor suppressor serpin that is encoded by the SERPINB5 gene. Playing an important role in embryonic development in cell adhesion, Maspin has the ability to suppress the growth, invasion, and metastasis of breast and lung tumors. However, Maspin does not exhibit any serine protease inhibitor activity since it does not undergo the conformational transition from stressed to relaxed found in typical serpins. It is expressed in normal breast and prostatic epithelial cells, but downregulated in most breast carcinoma.