Serine protease inhibitor. Expressed in liver. The reactive center loop (RCL) extends out from the body of the protein and directs binding to the target protease. The protease cleaves the serpin at the reactive site within the RCL, establishing a covalent linkage between the serpin reactive site and the active site of the protease. The resulting inactive serpin-protease complex is highly stable (By similarity). Belongs to the serpin family.