We identified a monoclonal KCNQ1 antibody able to normalize cardiac action potential duration and to suppress arrhythmias in a pharmacological model of LQTS type 3 using human-induced pluripotent stem cell-derived cardiomyocytes.<b>NEW & NOTEWORTHY</b> Long QT syndrome is a leading cause of sudden cardiac death in the young. This evidence concerns the gene KCNQ1 and Prolonged QT interval.