Acts downstream of ced-9 and caspase ced-3 to promote phosphatidylserine exposure on apoptotic cell surface, possibly by mediating phospholipid scrambling. Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment. Regulates apoptosis kinetics during embryonic development. Not required for engulfment of germ cell corpses. Cleavage by ced-3 activates ced-8 function in promoting phosphatidylserine exposure at the surface of apoptotic cells. Belongs to the XK family.