Calpains are a family of Ca²⁺-dependent intracellular cysteine proteases, including the ubiquitously expressed μ- and m-calpains. Both μ- and m-calpains are heterodimers, consisting of a distinct large 80-kDa catalytic subunit, encoded by the genesCapn1 and Capn2, and a common small 28-kDa regulatory subunit (Capn4). The physiological roles and possible functional distinctions of μ- and m-calpains remain unclear, but suggested functions include participation in cell division and migration, integrin-mediated signal transduction, apoptosis, and regulation of cellular control proteins such as cyclin D1 and p53. Homozygous disruption of murine Capn4 eliminated both μ- and m-calpain activities, but this did not affect survival and proliferation of cultured embryonic stem cells or embryonic fibroblasts, or the early stages of organogenesis. However, mutant embryos died at midgestation and displayed defects in the cardiovascular system, hemorrhaging, and accumulation of erythroid progenitors.