One of the proteins activated by the presence of calcium in the cell is calpain, a Ca2+-dependent non-lysosomal protease. About 15 minutes to half an hour after the onset of injury, a process called calpain-mediated spectrin proteolysis, or CMSP, begins to occur. Calpain breaks down a molecule called spectrin, which holds the membrane onto the cytoskeleton, causing the formation of blebs and the breakdown of the cytoskeleton and the membrane, and ultimately the death of the cell. Other molecules that can be degraded by calpains are microtubule subunits, microtubule-associated proteins, and neurofilaments.