SLC9A1 and cancer: For example, hypoxia induces genes involved in extracellular pH regulation, including monocarboxylate transporter 4 (MCT4) and sodium-hydrogen exchanger 1 (NHE1), as well as carbonic anhydrase 9 (CA9), which pump lactate and H+ ions out of cancer cells to maintain intracellular alkalinization and extracellular acidification [50].