Myoblast-specific protein that mediates myoblast fusion, an essential step for the formation of multi-nucleated muscle fibers. Actively participates in the membrane fusion reaction by mediating the mixing of cell membrane lipids (hemifusion upstream of MYMX. Acts independently of MYMX (By similarity. Involved in skeletal muscle regeneration in response to injury by mediating the fusion of satellite cells, a population of muscle stem cells, with injured myofibers (By similarity. Also involved in skeletal muscle hypertrophy, probably by mediating the fusion of satellite cells with myofibers (By similarity.