Calpains are calcium-dependent, cysteine-type endopeptidases that comprise MEROPS peptidase family C2. They play multiple roles in intracellular signal processing by limited proteolysis of target substrate proteins, thereby changing their function. Calpains are involved in numerous different physiological processes such as cell proliferation, migration, invasion, apoptosis and signal transduction. Compared to mammalian Calpains, D. melanogaster CalpA and CalpB are considered 'typical', CalpD (sol) has an atypical structure, while CalpC is a truncated form and is considered inactive because all three active site residues are mutated. (Adapted from FBrf0179911 and PMID:29693408.)