KCNH2 and familial long QT syndrome: Genotype–phenotype studies have shown that, in most cases, LQTS results from loss-of-function mutations in KCNQ1 (LQTS Type 1—LQT1) and KCNH2 (LQTS Type 2—LQT2) or gain of function in SCN5A mutations (LQTS Type 3—LQT3), all of which predispose young, otherwise healthy individuals to life-threatening arrhythmias [82,83,84,85,86].