Caspases are synthesized as inactive pro-enzymes that are processed to active form in cells undergoing apoptosis. Caspase-9 is an important member of the caspase family. Upon induction of apoptosis, Cytochrome c released from mitochondria associates with pro-caspase-9 (47 kDa) and Apaf-1. The complex processes pro-caspase-9 into a large subunit (37 kDa/17 kDa) and a small subunit (10 kDa). Cleaved caspase-9 further processes other caspases including caspase-3 and caspase-6, to initiate a caspase cascade leading to apoptosis. The affinity purified antibody recognizing the active forms of caspase-9 provides a new tool for identifying apoptotic cell populations in both tissue sections and cultured cells.