A
calpain (; , ) is a protein belonging to the family of
calcium-dependent, non-
lysosomal cysteine proteases (
proteolytic enzymes) expressed ubiquitously in mammals and many other organisms. Calpains constitute the C2 family of protease clan CA in the
MEROPS database. The calpain proteolytic system includes the calpain proteases, the small regulatory subunit
CAPNS1, also known as CAPN4, and the endogenous calpain-specific inhibitor,
calpastatin.