Cardiac arrhythmias affect approximately 5% of the population and have a high association with sudden death. Whilst the cause of cardiac arrhythmia is complex, there is a proven genetic component. Genomic sequencing of patients has contributed considerable information towards our understanding of gene mutations in this disease however genetic interpretation relies on our prior knowledge of gene function - typically derived from cell- and animal-based research. From a forward genetic screen in zebrafish, we have identified a completely novel gene required for cardiac rhythm. Mutant phenotypes include slower heart rates, skipped beats and 2:1 (atrium:ventricle) arrhythmias. Optical mapping of action potentials suggest that repolarization of the membrane is prolonged, suggesting an electrical defect in these mutants at the level of the action potential. Whole-genome sequencing mapping identified the causative mutation in a gene encoding a multi-transmembrane domain protein with no ascribed function and its closest homologue also has no known function. Spatial transcriptomics shows enrichment in the sinoatrial node (pacemaker) and atrioventricular node, consistent with the phenotype. We have now established a mouse knockout and find that mice die at birth. Earlier embryonic stages show enlarged hearts and poor blood flow consistent with impaired cardiac function. Importantly, in utero echocardiography imaging of 17.5 dpc embryos confirms cardiac arrhythmia in mutant embryos. Finally, sequencing information from patients with arrhythmogenic disorders reveal heterozygous mutations in the gene, consistent with it acting as a disease susceptibility locus. Together, this work describes a gene discovery project from zebrafish, through to mouse and finally humans, identifying a new regulator in cardiac rhythm.