Control,of the activation of apoptosis is important both in development and in protection against cancer. In the classic genetic model Caenorhabditis elegans,;the pro-apoptotic protein CED-4 activates the CED-3 caspase and is inhibited by the Bcl-2-like protein CED-9, Both processes are mediated by protein-protein interaction. Facilitating the proximity of CED-3 zymogen molecules was found to induce caspase activation and cell death. CED-4 protein oligomerized in cells and in vitro. This oligomerization induced CED-3 proximity and competed with CED-4:CED-9 interaction. Mutations that abolished CED-4 oligomerization inactivated its ability to activate CED-3, Thus, the mechanism of control is that CED-3 in CED-3:CED-4 complexes is activated by CED-4 oligomerization, which is inhibited by binding of CED-9 to CED-4.