Electrophysiological experiments are performed with human embryonic kidney (HEK) cells (whole-cell patch clamp) and Xenopus oocytes (double-electrode voltage clamp) expressing wild-type and mutant HERG channels. Ajmaline blocks HERG currents with an IC 50 of 1.0μM in HEK cells and 42.3μM in Xenopus oocytes.