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 (1) Mst2, a STE20-family member and purported Hpo ortholog, phosphorylates and activates both Lats1 and Lats2. Results indicate that Mst2-like kinases regulate Lats kinase activities in an evolutionarily conserved regulatory pathway (2). 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 (3).