NM_000218.3(KCNQ1):c.1552C>T (p.Arg518Ter) is a nonsense variant in exon 12 of 16 that introduces a premature stop codon at codon 518 and is predicted to trigger nonsense-mediated decay (PVS1). This variant is present in gnomAD v.4.1.0 at a maximum allele frequency of 0.0001737, with 205/1179992 alleles in the European (non-Finnish) population, which is lower than the ClinGen Potassium Channel Arrhythmia VCEP BS1 threshold of >0.0004 and higher than the PM2_Supporting threshold of <0.00001, so neither criterion is met. This variant has been detected in at least 2 apparently unrelated individuals with Jervell and Lange-Nielsen syndrome who had both a long QT interval and congenital deafness and harbored the variant in the homozygous state (1 pt, PMID: 29922582, PMID: 28438721, PM3), however PM3 is not met since this code requires the variant to be sufficiently rare (meeting PM2_Supporting). Additional individuals harbored the variant in the compound heterozygous state, some with autosomal recessive long QT syndrome (without congenital deafness) and others with Jervell and Lange-Nielsen syndrome (PMID: 24912595, PMID: 23392653). One individual harbored the variant in the compound heterozygous state, confirmed in trans with the NM_000218.3(KCNQ1):c.1573G>A (p.Ala525Thr) variant, which has not yet been classified by the ClinGen Potassium Channel Arrhythmia VCEP (PMID: 24912595). This variant has been shown to disrupt KCNQ1 function in at least four experimental assays, including manual patch-clamp and KCNQ1 mislocalization by immunofluorescence (PMID: 24912595, PMID: 25705178), however, PS3_Supporting is not met since the variant has already met PVS1. In summary, this variant meets the criteria to be classified as likely pathogenic for long QT syndrome 1 based on the ACMG/AMP criteria applied, as specified by the ClinGen Potassium Channel Arrhythmia VCEP: PVS1. (VCEP specifications version 1.0.0; date of approval 03/04/2025).