R25W reduces heteromeric peak tail and steady-state current density in human embryonic kidney (HEK) cells at physiological temperatures (36 ± 1°C). Sample current traces, normalized to cellular capacitance, and recorded from HEK cells coexpressing 1a/1b (A) or 1a/1b-R25W (B). (C) Peak tail current density measured at −50 mV, plotted as a function of pre-pulse potential, and fitted with a Boltzmann function for hERG 1a/1b channels (green) and hERG 1a/1b-R25W channels (blue). (D) Steady-state current measured at the end of 3-second test pulse, normalized to cellular capacitance and plotted as a function of test pulse potential for hERG 1a/1b (green) and hERG 1a/1b-R25W (blue). * indicates statistical significance compared to controls at p < 0.05. n = 6-9.