SCN5A and familial long QT syndrome: Over the past few decades, hundreds of LQTS-causing mutations have been identified in at least 15 LQTS-susceptible genes encoding cardiac ion channels or adaptor proteins.[8] Mutations in three major genes, including KCNQ1, KCNH2, and SCN5A, are responsible for nearly 80% of diagnosed cases,[9,10] whereas the mutations responsible for LQTS cases without these major ones remain elusive.