We knocked down CAII in normoxic (20% O2) and hypoxic (1% O2) MCF-7 breast cancer cells using siRNA and determined MCT-mediated proton–lactate co-transport by measuring changes in intracellular pH (pHi) during the application and the removal of 3 and 10 mM lactate in the nominal absence of CO2 and HCO3– (Figure 1A,B). The gene discussed is SLC16A1; the disease is breast carcinoma.