Rationale: Familial sinus node and atrioventricular conduction dysfunction is a rare disorder that leads to paroxysmal dizziness, fatigue, and syncope because of a temporarily or permanently reduced heart rate. To date, only a few genes for familial sinus and atrioventricular conduction dysfunction are known, and the majority of cases remain pathogenically unresolved. Objective: We aim to identify the disease gene in a large 3-generation family (n=25) with autosomal dominant sinus node dysfunction (SND) and atrioventricular block (AVB) and to characterize the mutation-related pathomechanisms in familial SND+ AVB. Methods and Results: Genome-wide linkage analysis mapped the SND+AVB disease locus to chromosome 7q21.1-q31.1 (2-point logarithm of the odds score: 4.64;.=0); in this region, targeted exome sequencing identified a novel heterozygous mutation (p. Arg52Leu) in the GNB2 gene that strictly cosegregated with the SND+ AVB phenotype. GNB2 encodes the beta(2) subunit (G beta(2)) of the heterotrimeric G-protein complex that is being released from G-protein-coupled receptors on vagal stimulation. In 2 heterologous expression systems (HEK-293T cells and Xenopus laevis oocytes), an enhanced activation of the G-protein-activated K+ channel (GIRK; Kir3.1/Kir3.4) was shown when mutant G beta(2) was coexpressed with G. 2; this was in contrast to coexpression of mutant G beta(2) -G. 2 with other cardiac ion channels (HCN4, HCN2, and Cav1.2). Molecular dynamics simulations suggested a reduced binding property of mutant G beta(2) to cardiac GIRK channels when compared with native G beta(2). Conclusions : A GNB2 gene mutation is associated with familial SND+ AVB and leads to a sustained activation of cardiac GIRK channels, which is likely to hyperpolarize the myocellular membrane potential and thus reduces their spontaneous activity. Our findings describe for the first time a role of a mutant G-protein in the nonsyndromic pacemaker disease because of GIRK channel activation.