In the presence of 10 mM tetraethylammonium (TEA) to fully inhibit endogenous potassium currents in tsA cells29 (Supplementary Fig. 1c)—different from the wild-type (WT+; with S620T) hERG channels30 - hERG channels with LQTS-associated mutation R531Q (R531Q+; also with S620T) demonstrated a noticeable initial phase of approximately 100 ms, preceding the delayed second phase (Fig. 1a). The gene discussed is KCNH2; the disease is familial long QT syndrome.