NM_000218.3(KCNQ1):c.477+5G>A is a variant in intron 2. The computational splicing predictor SpliceAI gives a score of 0.64 for donor gain (Threshold is > 0.5), predicting that the variant disrupts the donor splice site of intron 2 of KCNQ1 (PP3). A minigene assay showed that the variant disrupts splicing resulting in stop codon, indicating that this variant impacts protein function (PMID: 36197721), however at least two functional studies are needed for PS3_Supporting, so this criterion is not met. Another variant disrupting this splice site, NM_000218.3(KCNQ1):c.477+1G>A, has been classified as pathogenic for long QT syndrome 1 by the ClinGen Potassium Channel Arrhythmia VCEP, however, PS1 is not considered applicable for intronic variants. This variant has been reported in at least 13 probands with long QT syndrome (PS4; PMID: 19716085, PMID: 21350584, PMID: 23995044, PMID: 15840476, PMID: 10973849, PMID: 10728423). At least one affected proband exhibited QTc prolongation above 480 milliseconds and swimming-associated events, which together are highly specific for long QT syndrome 1 (PP4, PMID: 10560595). This variant has been reported to segregate with LQTS in 2 mother/child relationships, as well as one proband with JLNS and aunt affected with LQTS with QTc>480, which are not sufficient to meet the PP1 code (PMID: 23124029, PMID: 10728423). This variant is present in gnomAD v.4.1.0 at a maximum allele frequency of 0.00001337, with 1/74812 in the African / African-American population, which is higher than the ClinGen Potassium Channel Arrhythmia VCEP PM2_Supporting threshold of <0.00001, but lower than the BS1 threshold of >0.0004, so neither criterion is met. This variant has been detected in at least 2 individuals with a diagnosis of Jervell and Lange-Nielsen syndrome, one of whom harbored the variant confirmed in trans with the NM_000218.3(KCNQ1):c.1741A>T (p.Lys581Ter) variant, which was previously classified pathogenic by the ClinGen Potassium Channel Arrhythmia VCEP (PMID: 32508908), however PM3 is not met since this code requires the variant to be sufficiently rare (meeting PM2_Supporting). The other patient harbored the variant confirmed in trans with the p.Tyr171Ter variant, which was not specifically described at the DNA level and has not been previously classified by the ClinGen Potassium Channel Arrhythmia VCEP (PMID: 23392653). 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: PS4, PP3, and PP4. (VCEP specifications version 1.0.0; date of approval 03/04/2025).