Myoblast-specific protein that mediates myoblast fusion, an essential step for the formation of multi-nucleated muscle fibers (PubMed:23868259, PubMed:28386024, PubMed:28681861, PubMed:30197239). Actively participates in the membrane fusion reaction by mediating the mixing of cell membrane lipids (hemifusion) upstream of MYMX (PubMed:30197239). Acts independently of MYMX (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:25085416). Also involved in skeletal muscle hypertrophy, probably by mediating the fusion of satellite cells with myofibers (PubMed:28186492). Interacts with MYMX (PubMed:28386024). Localizes on the plasma membrane of myoblasts, where it mediates myoblasts fusion (PubMed:23868259, PubMed:28860190). Also localizes in the Golgi apparatus and post-Golgi following palmitoylation; the role of Golgi localization is unclear (PubMed:28860190). Specifically expressed in skeletal muscle during embryogenesis and adult muscle regeneration. During embryogenesis, highly expressed in the myotome compartment of the somites, and later in limb buds and axial skeletal muscles. Specifically expressed in skeletal muscle, and not in other muscle tissues or non-muscle tissues. Expression is down-regulated postnatally. Expression is induced in muscles in response to muscle injury (PubMed:25085416). Expression is induced in muscle progenitors response to muscle overload (PubMed:28186492). Down-regulated by in microRNA miR-491, which binds specifically to its 3' untranslated region of Mymk leading to its down-regulation (PubMed:28579197). Palmitoylated at the C-terminus; palmitoylation promotes localization to the Golgi apparatus. Perinatal death due to an absence of multi-nucleated muscle fibers (PubMed:23868259). Mice are observed at normal Mendelian ratios at 15 dpc and 17.5 dpc, full-term embryos are alive but are paralyzed and kyphotic with flaccid limbs due to skeletal muscle deficiency (PubMed:23868259). They show a complete absence of differentiated muscle tissue in the trunk, limbs or head (PubMed:23868259). Myoblasts can activate muscle-specific gene expression and differentiate, but lack the ability to fuse (PubMed:23868259). Defects are caused by impaired lipid mixing of cell membranes (PubMed:30197239). Conditional deletion in adult satellite cells, a population of muscle stem cells, completely abolishes muscle regeneration after injury, resulting in severe muscle destruction (PubMed:25085416). Conditional deletion in adult satellite cells impairs skeletal muscle hypertrophy in response to exercise (PubMed:28186492). Belongs to the TMEM8 family.