A small fragment of pre-existing glycogen must act as a primer to initiate glycogen synthesis. It is recently fount that in the absence of glycogen primer, a specific protein namely glycogenin can accept glucose from UDPG. The hydroxyl group of amino acid tyrosine of glycogenin is the site at which the initial glucose unit is attached. The enzyme glycogen initiator glucose synthase transfers the first molecule of glucose to glycogenin. Then glycogenin itself takes up a few glucose residues to form a fragment of primer, which serves as an acceptor for the rest of the glucose molecules.