The usefulness of targeting individual genetic mutations in cancer is limited by the huge variety of cancer genotypes.In contrast, the phenotypes of nearly all solid tumors include a high rate of lactate production, an intracellular pH (pHi) that is slightly alkaline and an extracellular pH (pHe) that, despite local variations, is generally acid.This distribution ofpH has three consequences of potential clinical importance.(1) Imaging pHe by magnetic resonance techniques may be a useful for diagnosis and for following the effects of therapy.(2) The pH gradient across the cell membrane can be exploited to target drugs.(3) Reducing the pH gradient by inhibiting tumor proton pumps slows tumor growth in animal models and may be of therapeutic value.