KCNH2 and long QT syndrome 3: The majority of patients are affected by the earliest of these identified: mutations in KCNQ1 (long QT syndrome 1; LQT1),3KCNH2 (long QT syndrome 2; LQT2),4,5 SCN5A (long QT syndrome 3; LQT3).6 Loss-of-function mutations in KCNQ1- encoded Kv7.1 channels and KCNH2-encoded Kv11.1 channels lead to a decrease in the slowly activating potassium channel (IKs) and rapidly activating potassium channel (IKr), respectively.