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. This evidence concerns the gene EDNRB and Hirschsprung disease.