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 caspase-3. Promotes DNA damage-induced apoptosis in a ABL1/c-Abl-dependent manner. Proteolytically cleaves poly(ADP-ribose) polymerase (PARP) (By similarity). Strict requirement for an Asp residue at position P1 and with a marked preference for His at position P2. It has a preferred cleavage sequence of Leu-Gly-His-Asp-|-Xaa. Heterotetramer that consists of two anti-parallel arranged heterodimers, each one formed by a 35 kDa (p35) and a 10 kDa (p10) subunit. Caspase-9 and APAF1 bind to each other via their respective NH2-terminal CED-3 homologous domains in the presence of cytochrome C and ATP. Interacts (inactive form) with EFHD2. Interacts with HAX1. Interacts with BIRC2/c-IAP1, XIAP/BIRC4, BIRC5/survivin, BIRC6/bruce and BIRC7/livin. Interacts with ABL1 (via SH3 domain); the interaction is direct and increased in the response of cells to genotoxic stress and ABL1/c-Abl activation (By similarity). Interacts with BCL2L10 (By similarity). Cleavages at Asp-353 by granzyme B and at Asp-368 by caspase-3 generate the two active subunits. Caspase-8 and -10 can also be involved in these processing events (By similarity). Phosphorylated at Thr-163 by MAPK1/ERK2. Phosphorylation at Thr-163 is sufficient to block caspase-9 processing and subsequent caspase-3 activation (By similarity). Phosphorylation on Tyr-191 by ABL1/c-Abl; occurs in the response of cells to DNA damage. Belongs to the peptidase C14A family.