SCN5A and familial long QT syndrome: As shown intable 2, about 70% of all the LQTS is caused by loss-of-function mutations of the Kv7.1(Iks) potassium channel (LQT1) or the Kv11.1 (IKr) potassium channel (LQT2) while 5%–10% is caused by gain-of-function mutations of the Nav1.5 (INa) sodium channel (LQT3).