Specific inhibition of calpain (calcium-dependent cysteine protease). Plays a key role in postmortem tenderization of meat and have been proposed to be involved in muscle protein degradation in living tissue. Each of the four flexible inhibitory domains can inhibit one calcium-bound calpain molecule by occupying both sides of the active site. In erythrocytes the protein lacks the N-terminal region of the liver inhibitor but retains three inhibitory units. Belongs to the protease inhibitor I27 (calpastatin) family.