SCN5A and familial long QT syndrome: Comment 2: Among clinically definite LQTS cases the three most frequently affected genes with a disease-causing change are KCNQ1, KCNH2 and SCN5A. Most of the LQTS patients are heterozygous for a variant affecting function, but in ~5% of the cases, patients carry two variants affecting function in the same gene (compound heterozygous or homozygous), or in different genes (digenic) [7, 8].