Muscle is not a simple and isolated organ, it constitutes approximately 70% of total body 31 mass and is arranged into bundles of multinucleated post mitotic fibres. Consequently, these 32 fibres rely on specific progenitors to facilitate development growth and repair (1). The 33 regulation and/or alteration of these progenitors have been widely studied in the context of 34 genetic muscle diseases and in relation to activation, proliferation and differentiation (2, 3). 35 Surprisingly, fusogenic processes remain broadly underrepresented in comparison and as 36 such, we have had to make some assumptions regarding myoblast fusion and resultant 37 myonuclear numbers. Membrane fusion is an essential biological process required for life. 38 During membrane fusion, two independent lipid membranes merge to create a singular 39 continuous bilayer, which facilitates: intracellular trafficking, organ formation and even 40 infection by enveloped viruses (4, 5). Cell-cell fusion is required for the development of 41 multicellular organisms via the joining of cells (as the name suggests) during formation of 42 organs and importantly fertilization (4). Conversely, aberrant fusion can exacerbate and 43 even cause a plethora of disorders including preeclampsia, cancer (6) and more recently 44 one genetic muscle disorder (7). The latter, named Carey Fineman Ziter Syndrome (CFZS), 45 is associated with mutations in the MYMK gene encoding for a protein essential for myoblast 46 fusion, myomaker (8). 47

48 Myomaker (mymk) is the first skeletal muscle specific protein identified as essential for 49 myoblast fusion (8, 9). Mymk knock out (KO) models produce mononucleated fibres in 50 zebrafish and mice which causes paralysis at birth in the latter (8, 10). Clinically, all CFZS 51 patients present with facial dysmorphisms and muscle hypotonia. To date, seven missense 52 mutations have been discovered (Fig. 1.). While two are yet to be categorised (c. 235T> C 53 and c. 399+ 5G> A) the classification of the remaining five was carried out via overexpression 54 in fibroblasts which were co-cultured with C2C12 myoblasts (7, 11-13). As no fusogenic 55 activity was observed in three of these mutations they were consequently characterised as 56 null (c. 553T4C, c. 298G4A and c. 2T4A). The final two mutations either; retained residual 57 fusion or displayed a reduced fusogenic capacity (c. 271C4A and c. 461T4C, respectively) 58 ultimately, resulting in the classification; hypomorphic. Unsurprisingly, the fusogenic 59 perturbation present in these individuals results in the formation of fewer myofibres when 60 compared to healthy individuals (11). These patients with Carey Fineman Ziter Syndrome 61 are a direct example of the involvement of myoblast fusion in the pathogenic roots of one 62 specific disease. What about other genetic muscle disorders? Is there an indirect disruption 63