It appears in Paint horses mostly, but can appear in any breed. The defect must be carried by both parents, and when it is the foal generally dies within 3 days of birth. The defect affects the intestinal track making it impossible for the foal to survive.
Horses can be carriers of this defective gene. A blood sample sent in for genetic testing can determine this. If you are planning to breed your Pinto horse (Paint especially), to an overo, be sure both parents are tested… mare and stallion. Responsible Paint breeders will usually have had their overos tested. Tobianos can carry the defective gene, too, so they should also be tested. If you choose not to have the horses tested, you are taking a chance, but the odds are about 50%.