Andersen-Tawil syndrome is characterized by periodic paralysis, ventricular ectopy, and dysmorphic features. Approximately 60% of patients exhibit loss-of-function mutations in KCNJ2, which encodes the inwardly rectifying K(+) channel pore forming subunit Kir2.1. Here, we report the identification of a novel KCNJ2 mutation (G211T), resulting in the amino acid substitution D71Y, in a patient presenting with signs and symptoms of Andersen-Tawil syndrome. The functional properties of the mutant subunit were characterized using voltage-clamp experiments on transiently transfected HEK-293 cells and neonatal mouse ventricular myocytes. Whole-cell current recordings of transfected HEK-293 cells demonstrated that the mutant protein Kir2.1-D71Y fails to form functional ion channels when expressed alone, but co-assembles with wild-type Kir2.1 subunits and suppresses wild-type subunit function. Further analysis revealed that current suppression requires at least two mutant subunits per channel. The D71Y mutation does not measurably affect the membrane trafficking of either the mutant or the wild-type subunit or alter the kinetic properties of the currents. Additional experiments revealed that expression of the mutant subunit suppresses native I(K1) in neonatal mouse ventricular myocytes. Simulations predict that the D71Y mutation in human ventricular myocytes will result in a mild prolongation of the action potential and potentially increase cell excitability. These experiments indicate that the Kir2.1-D71Y mutant protein functions as a dominant negative subunit resulting in reduced inwardly rectifying K(+) current amplitudes and altered cellular excitability in patients with Andersen-Tawil syndrome.