Cathepsin L is a lysosomal cysteine proteinase with a major role in intracellular protein catabolism. It also shows the most potent collagenolytic and elastinolytic activity in vitro of any of the cathepsins. Cathepsin L has been implicated in pathologic processes including myofibril necrosis in myopathies and in myocardial ischemia, and in the renal tubular response to proteinuria. Human liver cathepsin L consists of a heavy chain of about 25 kD and a light chain of about 5 kD. The gene is mapped to 9q21-q22. Cathepsin L is required for endothelial progenitor cell-induced neovascularization.