SCN5A and familial long QT syndrome: However, mutations in the KCNQ1, KCNH2, SCN5A genes, encoding the α subunit of the cardiac potassium channel Kv7.1, α subunit of the cardiac HERG potassium channel and α subunit of the cardiac sodium channel Nav1.5, respectively, are involved in about 75% of LQTS patients [56].