Serpin B9, also known as Cytoplasmic antiproteinase 3(CAP-3), is a cytoplasm protein which belongs to the large superfamily of serine proteinase inhibitors (serpins), which bind to and inactivate serine proteinases. Serpin B9 is an inhibitor of the granzyme B/perforin lytic pathway. It is expressed in normal mammary epithelial cells but not in most mammary carcinoma cell lines. These interactions are involved in many cellular processes, including coagulation, fibrinolysis, complement fixation, matrix remodeling, and apoptosis. Serpin-B9 expression in immune-privileged cells, APCs, and CTLs protects these cells against the actions of granzyme B, and when expressed in tumor cells or virally infected hepatocytes, confers resistance to killing by CTL and NK cells. Expression of increasing levels of Serpin-B9 in target cells may progressively inhibit immune surveillance by blocking NK and CTL-induced cytotoxicity through the perforin / granzyme pathway and then through the Fas / FasL pathway.