Lysosomal cysteine proteases, generally known as 'cathepsins', are optimally active in the slightly acidic milieu found in lysosomes. They comprise a group of papain-related enzymes, sharing similar amino acid sequences and folds. Most are endopeptidases, though some are aminopeptidases, aminodipeptidase or carboxydipeptidases. They are also extremely important processing enzymes and have several important specific and individual functions, often associated with restricted tissue localization. Cathepsins belong to MEROPS family C1, subfamily A. (Adapted from PMID:11532926.)