 The target population for the ABCD genetic sub-study includes all living individuals currently enrolled in the ABCD trial total study enrollment629; 566 analyzed in the trial Participants of the ABCD trial underwent TWA and EPS testing to determine their risk for ventricular arrhythmia and most have received an implantable cardioverter defibrillator ICD. The ABCD trial collected extensive longitudinal clinical data including medical and medication histories, incidence of ventricular arrhythmias, defibrillator shocks, other cardiovascular events, and mortality. 