GYG1 (glycogenin-1) is a critical enzyme that initiates glycogen biosynthesis by catalyzing the formation of short alpha(1,4)-glucosyl chains covalently attached to internal tyrosine residues, which serve as primers for glycogen synthase-mediated elongation 1. The enzyme functions through autoglucosylation to seed glycogen molecules and works in close coordination with glycogen synthase-1 (GYS1) in an asymmetric complex that facilitates handoff of the glycogen chain for subsequent elongation 1. GYG1 plays a fundamental role in cellular glucose homeostasis by regulating glycogen synthesis and breakdown in a cell-type-dependent manner, coordinating with GYG2 to modulate glycogen synthase activity 2. Mutations in GYG1 cause glycogen storage disease 15 and polyglucosan body myopathy 2, characterized by abnormal polyglucosan accumulation in muscle tissue 34. These disorders typically present with muscle weakness, atrophy, and in some cases cardiomyopathy with cardiac conduction defects 56. The polyglucosan bodies formed are amylopectin-like polysaccharides that are partially resistant to α-amylase digestion and have characteristic fibrillar appearance under electron microscopy 3. GYG1 deficiency represents one of the newly identified muscle glycogenoses that has expanded understanding of glycogen metabolism pathways 7.