CA9 and cancer: Interestingly, CAII and CAIX can facilitate MCT-mediated H+/lactate transport both in the inward and outward direction.114,180 During H+-coupled lactate efflux, as observed in lactate-producing cancer cells, intracellular CAII would collect H+ from surrounding protonatable residues near the inner face of the cell membrane and donate them to the transporter.