Emerging evidence shows that cysteine proteases have been implicated in the progression of tumors and in various critical tumor biological processes, including tumor cell aberrant proliferation and apoptosis, tumor cell-induced angiogenesis, as well as invasion of surrounding tissues and metastasis by malignant cells, suggesting that they are relevant to drug targets for treating cancer (Turk et al. 2004). Here, CTSB is linked to cancer.