Proteinase inhibitor 9 (PI-9, also designated cytoplasmic antiproteinase 3, or CAP3) is a 42 kDa member of the ovalbumin family of serpins that is expressed in placenta, lung and cytotoxic lymphocytes. PI-9 is a potent inhibitor of granzyme B and of granzyme B-mediated apoptosis, and is also an inhibitor of capase-1 and, to a smaller extent, caspase-4 and caspase-8. PI-9 expression is also upregulated in response to inflammatory stimuli. This upregulation protects cells from apoptosis induced by endogenously expressed or released granzyme B, particulary during target cell killing