(A) An illustrative example of a single cardiac cycle detected as spatial and temporal electrical gradients on the ECG. The P wave is generated by the spread of excitation through the atria. The QRS complex represents ventricular activation and is followed by the T wave, which reflects ventricular repolarization gradients. (B) Schematic representation of the KCNH2 (HERG) potassium channel α subunit, involving the N-terminal part (NH₃⁺), 6 membrane-spanning segments with the pore region located from segment S5 to segment S6, and the C-terminal portion (COO^–). Mutation locations are indicated by blue dots. Fourteen different mutations were located in 13 locations within the pore region. Reproduced with permission from Circulation (28).