Caspase 9 is a member of the Ced3/ICE cysteine protease caspase family that has a predicted molecular mass of 46kD and was originally isolated in human Jurkat T cells. Caspase 9 is critical for cytochrome c-dependent apoptosis and normal brain development. The active form of caspase 9 is composed of a 32-35kD subunit and a 10kD subunit that are proteolytically derived from procaspase 9 by caspase 3 (CPP3) and granzyme B. Procaspase 9 activation involves formation of a complex with the cofactor Apaf-1, cytochrome C, and deoxyadenosine triphosphate. Once activated, caspase 9 initiates a caspase cascade involving caspase 3,-6, and-7. Caspase 9 is expressed in a variety of tissues. The small heat shock protein Hsp27, has been shown to inhibit etoposide-induced apoptosis by preventing cytochrome C and dATP-triggered activity of caspase-9.