MONDO_0020290 (familial atrioventricular septal defect) is a spectrum of septal defects involving the atrial septum; ventricular septum; and the atrioventricular valves (tricuspid valve; bicuspid valve). These defects are due to incomplete growth and fusion of the endocardial cushions which are important in the formation of two atrioventricular canals, site of future atrioventricular valves. Also known as: Atrioventricular Septal Defect.