Here's a description in a nutshell:
Lethal White Overo (LWO) syndrome, also called overo lethal white syndrome (OLWS), occurs when a horse is homozygous (OO
) for the frame overo gene. This genetic disorder causes the intestinal system not to develop properly (involving aganglionosis of the bowel). The foal will die within the first 72 hours after birth when its first meals cannot be digested properly. The lethal white foal will be born almost pure white.
And a link to the entire article on the Animal Genetics Inc's site: LWO - Lethal White Overo Testing