Granzyme B is a serine protease found in the lymphocyte granules of cytotoxic T-lymphocytes (CTLs) and natural killer (NK) cells. Activated granzyme B prefers an aspartic acid residue at the P1 site of its substrates; a preference shared by only one other group of mammalian proteases, the caspases. Granzyme B is required for the rapid apoptotic signal delivered by CTLs and NK cells, an effect mediated by its ability to cleave and activate various procaspases. Granzyme B plays a crucial role in the development of acute graft-vs.-host disease. Thus, the inhibition of this enzyme may prove to be an important tool in controlling immune responses. Conversely, the failure of granzyme B-induced apoptosis may contribute to certain pathologies. Applications: Colorimetric detection, Fluorescent detection, HTS Activity assay.