EDNRB and Waardenburg syndrome type 4A: The lethal white foal syndrome (or overo lethal white syndrome, OLWS), which includes congenital aganglionosis, results from a defect in the EDNRB gene (118), and has been likened to a form of Hirschsprung disease in humans (186) that is associated with Waardenburg syndrome type 4A (WS4A) and that also results from an EDNRB mutation.