Myoblast-specific protein that mediates myoblast fusion, an essential step for the formation of multi-nucleated muscle fibers (PubMed:28386024, PubMed:28569745, PubMed:28569755, PubMed:30197239). Involved in membrane fusion downstream of the lipid mixing step mediated by MYMK (PubMed:30197239). Acts by generating membrane stresses via its extracellular C-terminus, leading to drive fusion pore formation (PubMed:30197239). Acts independently of MYMK (PubMed:30197239). 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 (PubMed:29581287). Interacts with MYMK (PubMed:28386024). Specifically expressed in developing skeletal muscles throughout the limbs and body wall (PubMed:28569745, PubMed:28569755, PubMed:28386024). Up-regulated during differentiation of myoblasts (PubMed:28569745, PubMed:28569755, PubMed:28386024). During muscle regeneration (PubMed:28569745, PubMed:28569755, PubMed:28386024). The AxLyCxL motif is required for myoblast fusion. Perinatal death due to a reduction in fused muscle fibers (PubMed:28569745). Embryos are motionless and lack skeletal muscle: they are nearly transparent so that internal organs and bones are apparent (PubMed:28386024). Myocytes differentiate and harbor organized sarcomeres but are fusion-incompetent (PubMed:28386024, PubMed:28569745, PubMed:28569755). Myoblast fusion is impaired downstream of membrane lipid mixing step mediated by Mymk and defects caused by the absence of Mymx are due to impaired fusion pore formation (PubMed:30197239). Conditional deletion in activated muscle stem cells (satellite cells) in adults abolishes satellite cell fusion and prevents muscle regeneration, resulting in severe muscle degeneration after injury (PubMed:29581287). Belongs to the MYMX family. Was initially reported to be specifically expressed in embryonic stem cells and germ cells (PubMed:16331322). However, it was later shown by different groups that it is specifically expressed in myoblasts where it promotes myoblast fusion (PubMed:28569745, PubMed:28569755, PubMed:28386024).