Ste20 is a S. cerevisiae Ser/Thr protein kinase that functions upstream of the MAP kinase module. Mammalian and yeast homologs of this kinase are divided into two classes based on their structure and regulation. Members of the first class (Ste20, Cla4, and p21-activated protein kinase [PAK]) contain a C-terminal kinase domain, an N-terminal regulatory domain and a small GTPase Rac1/Cdc42-binding domain. Members of the second class lack GTPase-binding sites, but are similar to the former class throughout the catalytic domain. The latter class includes GC kinase, HPK, KHS, KRS1 & 2, MST1, 2, & 3, and SOK-1. MST1 (Mammalian Sterile Twenty-like-1) is a ubiquitously expressed kinase that contains an N-terminal kinase domain and C-terminal dimerization and inhibitory domains. Apoptotic stimuli, such as anti-Fas, result in cleavage of the C-terminal regulatory domain by caspase-3 or a related caspase, and activation of MST1. Overexpression of MST1 induces caspase activity, apoptotic morphological changes, and the activation of the SAPK and p38 MAPK pathways. Thus, MST1 is thought to function as a component of a positive feedback loop that amplifies the apoptotic response.