Notably, Y662X, the most frequent KCNQ1 nonsense variant in gnomAD, showed gain-of-function, consistent with its detection in several patients with early-onset atrial fibrillation, syncope, or sudden death.40,63 Located in the last exon and only 42 bp from the native stop codon, Y662X may escape nonsense-mediated decay. This evidence concerns the gene KCNQ1 and atrial fibrillation.