Adult polyglucosan body disease (APBD) is a rare autosomal recessive glycogenosis caused by bi-allelic variants in GBE1. Impaired glycogen-branching enzyme (GBE) activity and upregulation of glycogen synthase cause the accumulation of polyglucosan bodies with detrimental effects on neurons and glia in the central and peripheral nervous system. The gene discussed is GBE1; the disease is glycogen storage disease due to glycogen branching enzyme deficiency.