During apoptosis, Apaf-1 (apoptosis protease activating factor 1) binds to cytochrome c (Apaf-2), a reaction that requires dATP, and this leads to the formation of a large multimeric complex. The complex recruits pro-caspase-9, and activates caspase-9 (Apaf-3). Activated caspase-9 in turn cleaves and activates caspase-3, one of the proteases responsible for the proteolytic cleavage of many key proteins in apoptosis. Apaf-1 is ubiquitously expressed in human tissues.