Potassium channels encoded by hERG (human ether-a-go-go-related gene) underlie the cardiac rapid delayed rectifier K+ current (I-Kr) and hERG Mutations underpin clinically important repolarization disorders. Virtually all electrophysiological investigations of hERG mutations have studied exclusively the hERG1a isoform; however, recent evidence indicates that native I-Kr channels may be comprised of hERG1a together with the hERG1b variant, which has a shorter N-terminus. Here, for the first time, electrophysiological effects were studied of a gain-of-function hERG mutation (N588K; responsible for the 'SQT1' variant of the short QT syndrome) on current (I-hERG1a/1b) carried by co-expressed hERG1a/1b channels. There were no significant effects of N588K on I-hERG1a/1b activation or deactivation, but N588K I-hERG1a/1b showed little inactivation up to highly positive voltages (<=+80 mV), a more marked effect than seen for hERG1a expressed alone. I-hERG1a/1b under action potential voltage-clamp, and the effects on this of the N588K mutation, also showed differences from those previously reported for hERG1a. The amplified attenuation of I-hERG inactivation for the N588K Mutation reported here indicates that the study of co-expressed hERG1a/1b channels should be considered when investigating clinically relevant hERG channel mutations, even if these reside outside of the N-terminus region. (C) 2009 Elsevier Inc. All rights reserved.