Over 1200 mutations of at least 15 LQTS-causative genes have currently been described (Table 5), but in 75% of cases the disease-causing mutation is found within three “traditional” genes (KCNQ1, KCNH2, SCN5A), accounting for up to 90% of positive genotypes. The gene discussed is KCNQ1; the disease is familial long QT syndrome.