The two main mammalian calpains, 1 and 2, are heterodimers of a large 80 kDa and a small 28 kDa subunit that together bind multiple calcium ions during enzyme activation. The main contact between the two subunits of these intracellular cysteine proteases is through a pairing of the fifth EF-hand of their C-terminal penta-EF-hand (PEF) domains.