Abnormal muscle fiber protein expression Abnormal muscle fiber calpain-3 UMLS:C4022626 Abnormal muscle fibre calpain-3 A deviation from normal in the amount of calpain-3 in muscle tissue. Calpains are intracellular nonlysosomal cysteine proteases modulated by calcium ions. A typical calpain is a heterodimer composed of two distinct subunits, one large (over 80 kDa) and the other small (30 kDa). While only one gene encoding the small subunit has been demonstrated, there are many genes for the large one. CAPN3 is similar to ubiquitous Calpain 1 and 2 (m-calpain and micro-calpain), but contains specific insertion sequences (NS, IS1 and IS2). Calpains cleave target proteins to modify their properties, rather than breaking down the substrates.