NM_000218.3(KCNQ1):c.217C>A (p.Pro73Thr) is a missense variant that causes substitution of proline with threonine at amino acid 73. This variant is present in gnomAD v.4.1.0 at a maximum allele frequency of 0.0002533, with 287 alleles / 1132988 total alleles in the European non-Finnish 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 is rare and has been reported in at least 4 apparently unrelated probands affected with long QT syndrome 1 (PS4_Moderate; PMID: 15840476, PMID: 19716085, PMID: 22949429, PMID: 24606995, PMID: 20851114). The variant has been reported to segregate with long QT syndrome 1 through a proband and 1 affected family member (PMID: 26743238), which is not sufficient to meet PP1. This variant has been observed in 1 patient with an alternate molecular basis for disease (NM_000335.5(SCN5A):c.1231G>A (p.Val411Met)) with a phenotype that matches long QT syndrome 3 (BP5; PMID: 28588847, PMID: 23098067). This variant has also been observed in 1 patient with additional variants in KCNQ1 present both in cis (NM_000218.3(KCNQ1):c.502G>A (p.Gly168Arg)) and in trans (listed as p.Asp454Thrfs*7) providing an alternate molecular basis for disease (PMID: 24667783), however, BP5 is only applicable when the phenotypes match another form of LQTS. The computational predictor REVEL gives a score of 0.566, which is below the ClinGen Potassium Channel Arrhythmia VCEP PP3 threshold of >0.75 but higher than the BP4 threshold of <0.25 and does not strongly predict a damaging effect on KCNQ1 function. The computational splicing predictor SpliceAI gives a score of 0.01 for donor gain, which is lower than the ClinGen Potassium Channel Arrhythmia VCEP PP3 threshold of >0.5 and does not strongly predict a damaging effect on KCNQ1 splicing. The Meiler Lab functional impact predictor (http://servers.meilerlab.org/servers/show?s_id=29) was unable to generate a prediction of functional impact for this variant due to a limitation of the model and the unavailability of secondary structure at this residue (PMID: 29021305), so neither PS3_Supporting nor BS3_Supporting was met. In summary, this variant meets the criteria to be classified as a variant of uncertain significance for long QT syndrome 1 based on the ACMG/AMP criteria applied, as specified by the ClinGen Potassium Channel Arrhythmia VCEP: PS4_Moderate, BP5. (VCEP specifications version 1.0.0; date of approval 03/04/2025).