Granzymes are neutral serine proteases, which are stored in specialized lytic granules of cytotoxic T lymphocytes (CTLs) and in natural killer (NK) cells. A number of granzymes (A to G) have been isolated and cloned from mouse CTLs and NK cells, however in man, fewer have been cloned and identified. One of these, human granzyme B, is involved in target cell apoptosis during lymphocyte-mediated cytotoxicity. Exocytosis of granzyme-containing granules in the cytoplasm of the target cell leads to the induction of DNA fragmentation and apoptosis.