Mouse Cathepsin L is a lysosomal cysteine proteinase which is a member of the peptidase C1 family. This protein is a dimer composed of disulfide-linked heavy and light chains, both produced from a single protein precursor. Cathepsin L plays a major role in intracellular protein catabolism. Its substrates include collagen and elastin, as well as alpha-1 protease inhibitor, a major controlling element of neutrophil elastase activity. Cathepsin L has been implicated in several pathologic processes, including myofibril necrosis in myopathies and in myocardial ischemia, and in the renal tubular response to proteinuria.