FLIP (FLICE inhibitory protein) is expressed as both long and short forms and is involved in the regulation of apoptosis. The short form of FLIP contains two death effector domains homologous to the death effector domain of the Fas-associating protein FADD. The long form of FLIP, which shares significant homology with the cysteine protease FLICE, contains an additional caspase-like domain, but lacks a catalytic active site and lacks the residues that form the substrate binding pocket in most caspases. FLIP has been designated by independent groups as Casper, I-FLICE, CLARP, FLAME-1 and MRIT. Although its exact role is still being elucidated, FLIP appears to be an important factor in the regulation of apoptosis downstream of all known death receptors.