Calpains are cytosolic, calcium-dependent, cysteine proteases that are ubiquitously expressed in mammalian and avian tissues. Calpain-1 and 2 consist of a common small subunit and a large variable subunit. Domains in the large subunit include the amino terminal domain I, the proteinase domain II, domain III, and the EF-hand domain IV. Type I calpain requires micromolar amounts of calcium, while type II calpain requires millimolar amounts of calcium.