channel has many signature features of the cardiac 'rapid' delayed rectifier current (IKr), including IKr's inward rectification that results in the channel producing a 'paradoxical resurgent current' in response to repolarization of the membrane. Structure A detailed atomic structure for hERG based on X-ray crystallography is not yet available, but structures have recently been solved by electron microscopy. In the laboratory the heterologously expressed hERG potassium channel comprises 4 identical alpha subunits, which form the channel's pore through the plasma membrane. Each hERG subunit consists of 6 transmembrane alpha helices, numbered S1-S6, a pore helix situated between S5 and S6, and cytoplasmically located N- and C-termini. The S4 helix contains a positively charged arginine or lysine amino acid residue at every 3rd position and is thought to act as a voltage-sensitive sensor, which allows the channel to respond to voltage changes by changing conformations between conducting and non-conducting states (called 'gating'). Between the S5 and S6 helices, there is an extracellular loop (known as 'the turret') and 'the pore loop', which begins and ends extracellularly but loops into the plasma membrane; the pore loop for each of the hERG subunits in one channel faces into the ion-conducting pore and is adjacent to the corresponding loops of the 3 other subunits, and together they form the selectivity filter region of the channel pore. The selectivity sequence, SVGFG, is very similar to that contained in bacterial KcsA channels. Although a full crystal structure for hERG is not yet available, a structure has been found for the cytoplasmic N-terminus, which was shown to contain a PAS domain (aminoacid 26–135) that slows the rate of deactivation. Genetics Loss-of-function mutations in this channel may lead to long QT syndrome (LQT2), while gain-of-function mutations may lead to short QT syndrome. Both clinical disorders stem from ion channel dysfunction (so-called channelopathies) that can lead to the risk of potentially fatal cardiac arrhythmias (e.g., torsades de pointes), due to repolarization disturbances of the cardiac action potential. There are far more hERG mutations described for long QT syndrome than for short QT syndrome. Drug interactions This channel is also sensitive to drug binding, as well as decreased extracellular potassium levels, both of which can result in decreased channel function and drug-induced (acquired) long QT syndrome. Among the drugs that can cause QT prolongation, the more common ones include antiarrhythmics (especially Class 1A and Class III), anti-psychotic agents, and certain