SCN5A and familial long QT syndrome: For example, the long QT syndrome (LQTS) has been considered classically an autosomal dominant genetic disorder, with heterozygous mutations in the three major LQTS-susceptibility genes (KCNQ1/LQT1, 30%–35%; KCNH2/LQT2, 25%–30%, and SCN5A/LQT3, 5%–10%) marked incomplete penetrance and variable expressivity [40–42].