[6][7][8][9] Cathepsin L1 is a member of the Peptidase C1 (cathepsin) MEROPS family, which plays an important role in diverse processes including normal lysosome mediated protein turnover, antigen and proprotein processing, and apoptosis.

[10] Its substrates include collagen and elastin, as well as alpha-1 protease inhibitor, a major controlling element of neutrophil elastase activity.

The encoded protein has been implicated in several pathologic processes, including myofibril necrosis in myopathies and in myocardial ischemia, and in the renal tubular response to proteinuria.

[11] Although Cathepsin L is usually characterized as a lysosomal protease, it can be secreted, resulting in pathological inflammation.

[14][15] Cathepsin L has been reported in many organisms including fish,[16] birds, mammals, and sponges.