CA9 and cancer: In this context, it might be interesting to compare the ability of bone sarcomas to acidify the extracellular space with respect to other types of cancers that are able to expand in bone, such as bone metastases (BM) (see ref. [50]); carcinoma cells metastasizing to the bone share with bone sarcoma cells different mechanisms of proton extrusion, including the expression of V1B2 and V0c V-ATPase subunits, CAIX, MCT1, and MCT4.