Myoblast-specific protein that mediates myoblast fusion, an essential step for the formation of multi-nucleated muscle fibers (PubMed:28386024, PubMed:29078404). Involved in membrane fusion downstream of the lipid mixing step mediated by mymk. Acts by generating membrane stresses via its extracellular C-terminus, leading to drive fusion pore formation (By similarity). Specifically expressed in the developing myotome. Expression coincides with myogenesis: expression is initiated between 12 and 14 hours post-fertilization (hpf), peaks at 19.5 hpf, gradually decreases thereafter and disappears at 36 hpf (PubMed:29078404). Specifically expressed in the developing somites, concomitant with muscle differentiation (PubMed:29078404). Present in all developing myotomes in 14-hpf (10 somites), 18-hpf (18-19 somites), and 19.5-hpf (21 somites) embryos (PubMed:29078404). As somitogenesis proceeds along the anteroposterior axis, expression gradually disappears from the differentiated anterior somites and shifts to the more caudal somites (PubMed:29078404). Not expressed in craniofacial muscles and the paraxial mesoderm (PubMed:29078404). The AxLyCxL motif is required for myoblast fusion. Abolished myoblast fusion. Belongs to the MYMX family.