Glycogen branching enzyme deficiency

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Glycogen Branching Enzyme Deficiency (GBED) is a genetic disease affecting horses, especially American Quarter Horses and related breeds. Lacking an enzyme necessary for storing glycogen, the horse's heart muscle and skeletal muscles cannot function, leading to rapid death.

The disease occurs in foals who are homozygous for the lethal GBED allele, meaning both parents must be heterozygous for the allele. A blood test for this allele was developed by the University of Minnesota College of Veterinary Medicine and as of 2005 is licensed to the UC Davis Veterinary Genetics Laboratory. Using this, breeders can avoid crosses that could produce GBED foals, and eventually selectively breed it out. Initial samples suggest that about 10% of Quarter Horses are carriers.