MGF is a split variant of IGF-1 but its sequence differs from the systemic IGF-1 produced by the liver. IGF-I is spliced towards MGF which initiates hypertrophy and repair of local muscle damage. MGF is expressed by mechanically overloaded muscle and is involved in tissue repair and adaptation. It is expressed as a pulse following muscle damage and is involved in the activation of muscle satellite (stem) cells. These donate nuclei to the muscle fibers that are required for repair and for the hypertrophy process, which may have similar regulatory mechanisms. MGF is essential for repair and therefore growth of new cells, similar to IGF-1. If MGF is not PEGylated, the half-life lasts only minutes therefore PEGylated MGF must be considered during the compounding process to ensure an appropriate half-life, thereby increasing duration of action.