Serine and cysteine protease inhibitor. Can inhibit lysosomal papain-like proteases including the cathepsins B, G, H, K, L and V. Ineffective against elastase, granzyme A, granzyme B, or caspases 3, 8 or 9. Inhibition of cytoplasmic cathepsin B following release from the lysosome may protect cells from apoptosis. This may facilitate the survival of progenitor T-cells and the subsequent development of long term memory CD8 T-cells. Expressed in bone marrow (particularly hematopoietic stem cells), heart, kidney, liver, lung, skeletal muscle, spleen, testis, thymus and T-cells. T-cell specific expression rises during the differentiation of CD8 T-cell progenitors into memory CD8 T-cells. Induction during apoptosis requires NF-kappa-B, a heterodimer of RELA- and NFKB1. 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 protease. The resulting inactive serpin-protease complex is highly stable (By similarity). Variability within the reactive center loop (RCL) sequences of Serpina3 paralogs may determine target protease specificity. The single human alpha1-antichymotrypsin gene (SERPINA3) is represented by a cluster of 14 individual murine paralogs. Belongs to the serpin family.