CA9 and glioblastoma: Membrane-bound carbonic anhydrases IX (CAIX) and/or XII (CAXII) colocalized with the membrane drug efflux protein, Pgp, in a range of drug resistant cancer cells including glioblastoma, maintain the intracellular/extracellular pH for efficient Pgp activity, and optimal tumor growth, invasion, and metastasis [175].