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. Isoform 2 is the major form in all tissues examined. Isoform 1 accounts for 5-10% in tissues such as skeletal muscle, liver and brain, and 30% in myoblasts. Isoforms 4 and 5 are testis-specific. Isoform 6 is highly expressed in heart and skeletal muscle with lower levels in liver, brain and testis. Isoform 7 is expressed at high levels in liver. Each of the four flexible inhibitory domains can inhibit one calcium-bound calpain molecule by occupying both sides of the active site. Belongs to the protease inhibitor I27 (calpastatin) family.