APBD is a genetic disorder that results in the low activity of an important enzyme, Glycogen Branching Enzyme (GBE1), which is used to make glycogen. What is glycogen? Glycogen is the reserve “fuel” that is stored in your cells. Stored glycogen normally takes over when your normal fuel, glucose, runs low. Glycogen itself is made by your body from glucose, but your cells must have a special enzyme called GBE1 to make it correctly. Without enough GBE1, APBD patients develop malformed glycogen