Involved in the activation cascade of caspases responsible for apoptosis execution. Binding of caspase-9 to Apaf-1 leads to activation of the protease which then cleaves and activates effector caspases caspase-3 (CASP3) or caspase-7 (CASP7). Promotes DNA damage- induced apoptosis in a ABL1/c-Abl-dependent manner. Proteolytically cleaves poly(ADP-ribose) polymerase (PARP). Cleaves BIRC6 following inhibition of BIRC6-caspase binding by DIABLO/SMAC (PubMed:36758105, PubMed:36758106). {ECO:0000269|PubMed:15657060, ECO:0000269|PubMed:16352606, ECO:0000269|PubMed:16916640, ECO:0000269|PubMed:23516580, ECO:0000269|PubMed:27889207, ECO:0000269|PubMed:35338844, ECO:0000269|PubMed:35446120}. [Isoform 2]: Lacks activity is an dominant-negative inhibitor of caspase-9. . This is the function of Ensembl gene identifier ENSG00000132906 (CASP9, caspase 9).