Anti-apoptotic protein which acts by inhibiting the activities of CASP3, CASP7 and CASP9. Can inhibit the autocleavage of pro-CASP9 and cleavage of pro-CASP3 by CASP9. Capable of inhibiting CASP9 autoproteolysis at 'Asp-315' and decreasing the rate of auto proteolysis at 'Asp-330'. Acts as a mediator of neuronal survival in pathological conditions. Prevents motor-neuron apoptosis induced by a variety of signals (By similarity). Interacts (via NACHT domain) with APAF1 (via CARD and NACHT domains). Both the BIR and NACHT domains are essential for effective inhibition of pro-CASP9 cleavage. BIR3 domain binds to procaspase-9 and the NACHT domain interacts with the NACHT domain of APAF1 forming a bridge between pro-CASP9 and APAF1 (By similarity).