CTSK and melanoma: Indeed, apart from its well-confirmed involvement in bone disease, cathepsin K is also highly expressed in gastric cancer, squamous cell carcinoma, basal cell carcinoma, breast tumors, lung cancer, melanomas, prostate tumors, and renal tumors and appears to have a positive role in promoting tumor progression.[9,10] Other cathepsins (including cathepsins B, D, L, etc) also have the potential to accelerate tumorigenesis by promoting extracellular matrix (ECM) degradation.