In humans, two isoforms of glycogenin can be expressed: glycogenin-1, with a molecular weight of 37 kDa and codified  by GYG1 gen, which is expressed mostly in muscles;  and glycogenin-2, with a molecular weight of 66 kDa and codified by GYG2 gen, which is expressed mainly in liver, cardiac  muscle and other types of tissue, but not in skeletal muscle.
Glycogenin-1 was described by analyzing the glycogen of skeletal muscle. It was determined that this molecule was united by a covalent bond to each mature molecule of muscular glycogen.