To circumvent the abovementioned issues, Wu and Gale recommended a modified HPTS transport assay (Figure , a, termed as the 'anion gradient' assay) for studying anion selectivity by switching the external solution for a solution of the anion of interest 'X -' (NaX, 100 mM) without adding a base pulse. The driving force of the observed transport is the transporter-induced membrane potential arising from different transport rates of Cl -and X -that dictates the direction and magnitude of H + (or OH -) transport.