Plays a major role in programmed cell death (PCD or apoptosis) by negatively regulating ced-9 (PubMed:9604928, PubMed:10688797, PubMed:15383288). Binds to and directly inhibits the activity of ced-9, releasing the cell death activator ced-4 from a ced-9/ced-4 containing protein complex and allowing ced-4 to activate the cell-killing caspase ced-3 (PubMed:9604928, PubMed:10688797, PubMed:15383288). Required to activate programmed cell death in the sister cells of the serotonergic neurosecretory motor (NSM) neurons during embryogenesis (PubMed:12874127). Required to activate programmed cell death in the sister cells of the M4 motor neuron and I1 pharyngeal neuron during embryogenesis (PubMed:20713707). During larval development, required for the elimination of transient presynaptic components upstream of ced-9, ced-4 and ced-3 apoptotic pathway (PubMed:26074078). Together with ain-1, a component of the miRNA-induced-silencing complex (miRISC), and probably upstream of ced-3 and ced-4, regulates temporal cell fate patterning during larval development (PubMed:25432023). Interacts with ced-9; the interaction results in ced-4 release from the ced-4/ced-9 complex. Localizes to RMED/V synaptic regions in L1 larvae. Mutants block programmed cell death (PubMed:9604928). In an ain-1 mutant background, enhances the proportion of animals arrested at the larval stage, with egg-laying defects and with a ruptured vulva (PubMed:25432023). Truncated N-terminus.