Andersen-Tawil syndrome (ATS) is a multisystem inherited disease exhibiting periodic paralysis, cardiac arrhythmias, and dysmorphic features. In this study, we characterized the KCNJ2 channels with an ATS mutation (T75M) which is associated with cardiac phenotypes of bi-directional ventricular tachycardia, syncope, and QT(c) prolongation. Confocal imaging of GFP-KCNJ2 fusion proteins showed that the T75M mutation impaired membrane localization of the channel protein, which was restored by co-expression of WT channels with T75M channels. Whole-cell patch-clamp experiments in CHO-K1 cells showed that the T75M mutation produced a loss-of-function of the channel. When both WT and the T75M were co-expressed, the T75M mutation showed dominant-negative effects on inward rectifier K+ current densities, with prominent suppression of outward currents at potentials between 0 mV and +80 mV over the E(K). Inside-out patch experiments in HEK293T cells revealed that co-expression of WT and the T75M channels enhanced voltage-dependent block of the channels by internal Mg2+, resulting in enhanced inward rectification at potentials 50 mV more positive than the E(K). We suggest that the T75M mutation causes dominant-negative suppression of the co-expressed WT KCNJ2 channels. In addition, the T75M mutation caused alteration of gating kinetics of the mutated KCNJ2 channels, i.e., increased sensitivity to intracellular Mg2+ and resultant enhancement of inward rectification. The data presented suggest that the mutation may influence clinical features, but it does not directly show this.