CTSB and neoplasm: Extracellular pH in bones is associated with osteoblast and osteoclast function and with bone acidification leading to enhanced osteoclast resorption.64 Metastatic tumor cells in localized bone regions produce lactic acid, resulting in acidosis of the bone microenvironment.64 Tumor acidosis, in turn, increases the secretion of proteins that degrade the ECM and thus facilitate metastases, such as cathepsin B and matrix metallopeptidases (MMPs).65