Approximately 80% of LQTS stems from either loss-of-function (LOF) or gain-of-function (GOF) pathogenic variants in 1 of 3 LQTS-susceptibility genes: KCNQ1-encoded IKs (Kv7.1) potassium channel (LQTS type 1 [LQT1], ∼35%–40%, LOF), KCNH2-encoded IKr (Kv11.1) potassium channel (LQTS type 2 [LQT2], ∼30%–35%, LOF), or SCN5A-encoded INa (Nav1.5) sodium channel (LQTS type 3 [LQT3], ∼5%–10%, GOF). This evidence concerns the gene INA and familial long QT syndrome.