Cathepsin V (CTSU, CTSV, CATL2, cathepsin U, cathepsin L2) is a lysosomal cysteine proteinase that influences corneal physiology, and mediates degradation of invariant chain in human thymus. Cathepsin V is present in corneal epithelium, activated macrophages, and colorectal and breast carcinomas. A 1.8-kb mRNA to Cathepsin V is present in normal human thymus and testis. Cathepsin V is a member of the peptidase C1 family. Cysteine proteinases are synthesized as preproenzymes, which are processed to the corresponding proenzymes. The proenzymes are either targeted to the lysosome or continue along the cellular secretory route.