Most (74%2) biallelic KCNQ1 variants leading to JLNS1 include at least one truncating variant which may interfere with or prevent channel assembly, while most of heterozygous nucleotide substitutions in KCNQ1 leading to LQTS are missense variants with a dominant-negative effect, with incomplete penetrance7, since they are expected to co-assemble with normal subunits and interfere, to a variable extent, with channel function2,8. This evidence concerns the gene KCNQ1 and familial long QT syndrome.