Granzyme family serine proteases are stored in the granules of cytotoxic T lymphocytes and natural killer cells. Granzymes contain one S1 protease domain and are synthesized as preproproteins. Granzymes are released toward pathogen infected or transformed target cells during cellular immune reactions. Target cell entry by granzymes is through perforin channels. Granzymes trigger apoptosis by caspase-dependent and -independent mechanisms. There are five granzymes (A, B, G, H and K) in human and many more in mouse and rat.