Cytosolic caspase 3 cleaves p-STK4 (p-MST1) to yield an active animo-terminal fragment (p-STK4/N) and a carboxy-terminal fragment (p-STK4/C) (Graves et al. 1998; Lee et al. 2001). The association of p-STK4 (p-MST1) with other proteins at the time of its cleavage by caspase has not been studied experimentally. Here, it is inferred to be dimerized and in a complex with SAV1 because that is the form of the molecule that becomes phosphorylated and phosphorylation appears normally to precede caspase cleavage. The effect of the cleavage is to increase the kinase activity of p-STK4 (p-MST1).