By adding acid or base containing the anion 'X -' of interest to calibrate the bulk internal and external solutions containing NMDG (100 mM) to pH 7.0, the NMDG-X salt is formed. Lastly, to determine the Cl -vs H + /OH -transport selectivity (providing the transporter being tested can facilitate anion-selective transport (uniport)), the HPTS assay is conducted with either valinomycin (Vln), a potassium-selective ionophore that transports K + down an electrochemical gradient, or FCCP (CCCP or Gra with NMDG-Cl, Figure and). The transport observed in these systems can be compared to a control experiment using only the transporter to facilitate transport (Figure ). It should be noted that several anion transporters have been reported to be inactive in facilitating uniport processes due to strong interactions with the phospholipid headgroup, which inhibits the free transporter diffusion. These anion transporters are unsuitable for these experiments because coupling to either Vln or FCCP (or Gra) requires a uniport mechanism. Compared to the control experiment, enhanced transport activity seen during the Vln treated experiment indicates Cl -< H + /OH -selectivity of the anion transporter as the Vln replaces the rate-limiting Cl -transport step through the rapid transport of the K + counter cation. This type of anion transporter does not respond to the proton transporter FCCP (or Gra) because the rate-limiting process, H + transport, is not improved. On the other hand, Cl -> H + /OH -anion transporters demonstrate enhanced transport in the presence of protonophore FCCP and insensitivity to Vln. Insensitivity to both FCCP and Vln might indicate a 'non-uniporting' mechanism or similar H + and Cl - transport rates.