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. This evidence concerns the gene CTSK and squamous cell carcinoma.