NM_000218.3(KCNQ1):c.590C>T is a missense variant predicted to cause replacement of proline with leucine at position 197. This residue is conserved across the 5 human KCNQ paralogues (https://www.cardiodb.org/paralogue_annotation/gene.php?name=KCNQ1) and another missense variant in the same codon, NM_000218.3(KCNQ1):c.589C>T (p.Pro197Ser), has been investigated in connection with long QT syndrome 1 (PMID: 29532034) but has been classified as a variant of uncertain significance for long QT syndrome 1 by the ClinGen Potassium Channel Arrhythmia VCEP, so PM5 is not yet met. This variant is present in gnomAD v.4.1.0 at a maximum allele frequency of 0.00004576, with 54 / 1179994 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 has been reported in the cardiovascular disorder literature, however, the available clinical details are not sufficient for inclusion in PS4, so the PS4_Supporting code is not yet met. The computational predictor REVEL gives a score of 0.965, which is above the ClinGen Potassium Channel Arrhythmia VCEP PP3 threshold of >0.75 and predicts a damaging effect on KCNQ1 function (PP3). The variant demonstrates 60-70% of wild-type cell surface trafficking by flow cytometry (PMID: 29532034), but 140-150% of wild-type peak current density (PMID: 29532034, PMID: 30571187), which is not compatible with a proposed role as a disease-causing variant for long QT syndrome 1, so the PS3 code is not 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: PP3. (VCEP specifications version 1.0.0; date of approval 03/04/2025).