Loss-of-function variants in KCNQ1 are the most common cause of the congenital long QT syndrome (LQTS).1 LQTS is characterized by QT prolongation on the electrocardiogram, which confers an increased risk of potentially fatal arrhythmias.2,3 Most KCNQ1-related LQTS cases arise in heterozygous carriers of loss-of-function variants. This evidence concerns the gene KCNQ1 and familial long QT syndrome.