Somatic mosaicism, the occurrence and propagation of genetic variation in cell lineages after fertilization, is increasingly recognized to play a causal role in a variety of human diseases. We investigated the case of life-threatening arrhythmia in a 10-day-old infant with long QT syndrome (LQTS). Rapid genome sequencing suggested a variant in the sodium channel Na V 1.5 encoded by SCN5A, NM_000335:c.5284G > T predicting p.(V1762L), but read depth was insufficient to be diagnostic. Exome sequencing of the trio confirmed read ratios inconsistent with Mendelian inheritance only in the proband. Genotyping of single circulating leukocytes demonstrated the mutation in the genomes of 8% of patient cells, and RNA sequencing of cardiac tissue from the infant confirmed the expression of the mutant allele at mosaic ratios. Heterologous expression of the mutant channel revealed significantly delayed sodium current with a dominant negative effect. To investigate the mechanism by which mosaicism might cause arrhythmia, we built a finite element simulation model incorporating Purkinje fiber activation. This model confirmed the pathogenic consequences of cardiac cellular mosaicism and, under the presenting conditions of this case, recapitulated 2:1 AV block and arrhythmia. To investigate the extent to which mosaicism might explain undiagnosed arrhythmia, we studied 7,500 affected probands undergoing commercial gene-panel testing. Four individuals with pathogenic variants arising from early somatic mutation events were found. Here we establish cardiac mosaicism as a causal mechanism for LQTS and present methods by which the general phenomenon, likely to be relevant for all genetic diseases, can be detected through singlecell analysis and next-generation sequencing.here is growing recognition that somatic mosaicism, i.e., genetic variation within an individual that arises from errors in DNA replication during early development, may play a role in a variety of human diseases other than cancer (1). However, the extent to which cellular heterogeneity contributes to disease is minimally understood. One report suggests that 6.5% of de novo mutations presumed to be germline in origin may instead have arisen from postzygotic mosaic mutation events (2), and recent genetic investigations directly interrogating diseased tissues in brain malformations, breast cancer, and atrial fibrillation have revealed postzygotic causal mutations absent from germline DNA (3-6). Pathogenic mosaic structural variation is also detectable in children with neurodevelopmental disorders (7). However, a consequential category of genetic variation has not been surveyed systematically in clinical or research studies of other human diseases.The pathophysiological basis of long-QT syndrome (LQTS) is prolongation of cardiac ventricular repolarization by acquired factors such as drug exposure or genetic variation in the proteins controlling transmembrane ion-concentration gradients (8, 9); parental gonadal mosaicism is an infrequently described phenomenon in LQTS (10...