Glycogen branching enzyme deficiency (GBED) is a fatal, autosomal-recessive disease that occurs in newborn Quarter horse, Paint horse, and related breeds' foals. It is caused by Y34X missense mutation at codon 34 in exon 1 of the glycogen branching enzyme 1 (GBE1) gene. The foals lack the enzyme necessary to store glycogen (sugars) in its branched form and therefore cannot store sugar molecules. The foal is born lacking the enzyme necessary for storing glycogen and the horse's brain, heart muscle, and skeletal muscles cannot function, leading to rapid death.

It is estimated that 8% of Quarter Horses and Paint Horses are carriers of the mutation. Since the gene is recessive, a horse can carry the gene and pass it to its offspring unknowingly. GBED is related to Glycogen storage disease type IV in humans.