Abstract: This invention relates to a novel calpain having a proteolytic activity, its partial peptide or a salt either of them, a DNA coding for the protein, a recombinant vector comprising the DNA, a transformant carrying the recombinant vector, a process for producing the protein, a pharmaceutical composition comprising the DNA, an antibody against the protein, a method for screening for a compound which activates or inhibits a proteolytic activity of the protein, a kit for screening for the compound, and a compound which activates or inhibits a proteolytic activity of the protein which is identified by the screening method or the kit. The DNA coding for the protein of the present invention can be used as a therapeutic and prophylactic composition for a variety of diseases including tumor, cerebral apoplexy, cerebral infarction, subarachnoid hemorrhage, Alzheimer's disease, myodystrophy, cataract, ischemic heart disease, atherosclerosis, arthritis, and collagen disease.