Cathepsin L is a normal lysosomal cysteine endoproteases that is expressed in all cells. The latent 39 kDa pro-Cathepsin L must be processed by cleavage of the 96 amino acid pro-peptide, to generate the active two-chain form made up of 21 and 5 kDa subunits. Mature Cathepsin L is normally localized in the lysosomes where it plays a major role in intracellular protein catabolism. The fact that Cathepsin L is also capable of degrading extracellular matrix proteins sμggests that it may play a role in invasion and metastasis and could be a useful prognostic indicator for cancer.