Quantification of relative Cs⁺/Cl⁻ permeability in HEK cells transfected with dBest1. Whole cell current was measured in wild-type dBest1 or dBest1-F81C–overexpressed HEK cells. Changes in E_rev (ΔE_rev) as a function of extracellular CsCl concentration were plotted for cells with and without MTSES⁻ treatment. Each data point represents the mean E_rev ± SEM of two to seven cells. Gray dashed line was calculated from the GHK equation assuming that the channel is exclusively permeable to Cl⁻ (P_Cs/P_Cl = 0). Filled symbols: MTSES⁻-treated; •, F81C (n = 2–7); ▪, wild-type dBest1 (n = 3–4). Open symbols: without MTSES⁻ treatment; ○, wild-type dBest1 (n = 5); □, F81C (n = 3).