Lethal white overo syndrome (LWOS) is an inherited, autosomal congenital condition affecting newborn American Paint Horse foals. It occurs when foals are born homozygous for the lethal white gene. It is the result of breeding two carrier (Overo) horses. LWOS foals are born completely white with blue eyes. Foals are born with ileocolonic aganglionosis, which results in colon dysmotility, colic and ultimately fatal.