Complete absence of IKs in individuals with homozygous or compound heterozygous pathogenic variants in KCNQ1 leads to LQTS combined with deafness (Jervell-Lange-Nielsen syndrome), while individuals harboring heterozygous pathogenic variants in KCNQ1 display LQTS with normal hearing. Here, KCNQ1 is linked to familial long QT syndrome.