white syndrome (OLWS or LWS), the equine version of Hirschprung disease. LWS occurs when a foal is homozygous for the Ile118Lys EDNRB mutation, which is considered by many researchers to be "usually responsible" for the frame overo phenotype. However, other researchers emphasize that overo spotting patterns are phenotypically and genetically heterogeneous, that is, may have multiple sources. The frame overo gene can be masked by other white patterns, particularly tobiano, which is a dominant gene and epistatic to overo. Epistatic means that when both genes are present, this is the one expressed. In addition, some carriers of the LWS allele appear to be solid. One theory holds that such horses carry the frame gene, but so minimal in expression that they appear solid. Either way, all LWS foals have horses with frame overo patterning in their pedigrees, and horses carrying a frame allele may not necessarily have a visible expression of the frame overo color. "Lethal white" Foals which are homozygous for frame and thus have lethal white syndrome (LWS) are not albinos. Albinism does not exist in horses. LWS foals are born almost or completely white with pink skin, but have blue eyes, not red ones. The lethal trait is that the nerves of the foal's digestive system are undeveloped and the bowel cannot move food along. All lethal white foals die within 72 hours after birth, and are typically euthanized sooner for humane reasons. A horse can be a carrier of the trait when it carries only one copy of the gene, (that is, heterozygous) and carriers are healthy and show no clinical signs of lethal white syndrome. Thus, it is recommended by organizations such as the American Paint Horse Association that horses of any pattern with overo ancestors be tested to verify their carrier state before being bred. There is a DNA test for the gene which can be used to plan breedings and avoid producing affected foals. The mutation has not been found in solid-colored horses from breeds without frame patterning. Likewise, a sick foal that happens to be white can be tested to avoid inadvertent euthanization of a non-lethal foal who has a simple case of colic. Lethal white horses should not be confused with non-lethal forms of white, such as dominant white, which may be completely white with pink skin. Blue-eyed, pink-skinned cremellos and other horses carrying the cream gene do not carry the