Failure of single ventricle patients to progress through the multi-stage palliation or late failure of patients after the Fontan procedure can result from a myriad of causes. While ventricular dysfunction is a common finding in failing single ventricle patients, an anatomic reason for their clinical failure, such as myocardial ischemia or ventricular outflow obstruction, often cannot be identified. Many of these patients subsequently die or require cardiac transplantation when their single ventricle fails. Because patient outcomes are highly variable despite similar anatomy and hemodynamics, it has been suggested that genetic variability may play a role in the ability of patients to tolerate long-term single ventricle palliation.