The “human ether-à-go-go”-related gene (hERG) encodes for the alpha subunit of the cardiac rapid delayed rectifier current (IKr) channel that modulates action potential duration and is involved in the long-QT type 2 syndrome. A naturally occuring mutation causes the replacement of a phenylalanine by a leucine at position 627 (p.F627L) [1,2]. This residue is located in the highly conserved GFG motif in ERG channels, homologous to the GYG selectivity motif in other potassium-selective channels. We investigated the kinetics and ion selectivity of channels made of F627L mutated subunits versus channels made of wild type (WT) subunits.Xenopus oocytes were microinjected with 200 ng capped mRNA encoding either WT hERG or F627L mutant hERG. Currents were recorded after 3 to 5 days with dual electrode voltage clamp at 22+-1°C. Methods were as in [3].In oocytes expressing F627L channels the slope-conductance at -30 mV was decreased 2-fold versus WT channels, and inward rectification was abolished. The voltage for half steady-state activation was shifted to -42.9±2.1 (n=9) for F627L versus -36.8±0.8 (n=8) for WT (p<0.05, Student’s unpaired t-test), without change in the time constant of activation.No time-dependent inactivation of the F627L current was seen in response to a classical 3-pulse protocol. Time-dependent deactivation was absent at voltages positive to -80 mV while fast deactivation below -80 mV was preserved. In normal Ringer solution, the reversal voltage (Vrev) of the F627L current was shifted positively (p<0.01) at -38.8±8.1 mV (n=8) versus -88.5±2.2 mV (n=6) for the WT current. Vrev changes with [K+]o or [Na+]o (replaced by N-methyl D-glucamine: NMDG+) were fitted by the Goldman-Hodgkin-Katz equation. A permeability ratio PNa/PK=0.27 for the F627L current, versus 0.03 for the WT, accounted for [K+]o related changes. For fitting Vrev changes versus [Na+]o, a sizable permeability to NMDG+ had to be added (PNMDG/PK=0.17) to a PNa/PK=0.35. Sensitivity of the F627L current to 5.8 µM E4031 and 1 µM dofetilide was decreased 9-fold and 7-fold, respectively, versus the WT current.Thus, unlike in the GYG motif of Kv potassium channels, replacement of the aromatic side-chain by a neutral one in the central residue of the GFG motif of the hERG channel strongly affected altogether inactivation, deactivation, selectivity and drug sensitivity. The physico-chemical mechanism(s) of these effects and their physiopathological consequences remain to be clarified.