The human serine/threonine kinase, mammalian STE20 like kinase (MST), is considerably homologous to the budding yeast kinases, SPS1 and STE20, throughout their kinase domains. When stably expressed in HeLa cells, MST highly sensitizes the cells to death receptor mediated apoptosis by accelerating caspase 3 activation. These findings suggest that MST1 and MST2 play a role in apoptosis both upstream and downstream of caspase activation. MST1 is cleaved and activated by caspases during apoptosis and is capable of inducing apoptotic morphological changes such as chromatin condensation upon overexpression. Mammalian Sterile 20 Like 2 (MST2) is most similar to the previously identified MST1 protein kinase (78% identity, 88% similarity). Northern analysis indicates that MST2 mRNA is expressed at high levels in adult kidney, skeletal and placental tissues and at very low levels in adult heart, lung, liver and brain tissues. An in vitro kinase assay indicates that MST2 can phosphorylate an exogenous substrate, as well as itself, and phospho amino acid analysis indicates that it is a serine/threonine protein kinase.