The mammalian homologues of the key cell death gene CED 4 in C. elegans has been identified recently from human and mouse and designated Apaf1 (for apoptosis protease activating factor 1). Apaf1 binds to cytochrome c (Apaf2) and caspase 9 (Apaf3), which leads to caspase 9 activation. Activated caspase 9 in turn cleaves and activates caspase 3 that is one of the key proteases, being responsible for the proteolytic cleavage of many key proteins in apoptosis. A new Apaf1 Interacting Protein (APIP) also known as CG129 and MMRP19, has been identified as a negative regulator of ischemic injury. APIP competes with Caspase 9 binding site of Apaf1. APIP is predicted to code for a 204 amino acid. An isoform of APIP, APIP2 encodes a 242 amino acid protein, which is an alternative splicing variant differing in its N terminus from APIP. APIP transcript is ubiquitously expressed in most adult tissue with high expression in skeletal muscle, heart, and kidney.