SLC9D1 and neoplasm: TMCO3 is a member of monovalent cation: proton antiporter 2 (CPA2) family of transporter proteins.[3] Members of this family typically couple the export of monovalent cations, such as potassium or sodium, to the import of protons across cellular membranes.[4] Members of this family have been poorly studied in cancer area, and tumor research on TMCO3 have been negligible.