SKELETAL muscle development involves the formation of multinucleated myotubes. This is thought to proceed by the induction of differentiation (acquisition of fusion competence) of myoblast cells, their aggregation, and union of their plasma membranes(1-3) Various membrane proteins including N-(4,5) and M-cadherins(6), N-(5,7-9) and V-CAMs(10) and integrins(10,11) participate in myotube formation, but the molecular mechanisms of muscle cell fusion are poorly understood. Here we report the identification of three new, myoblast-expressed gene products, mcltrin-alpha, beta and gamma, with homology to both viper haemorrhagic factors(12,13) and fertilin (PH-30)(14,15), a membrane protein involved in egg-sperm fusion. Meltrin-alpha, a member of the metalloproteinase/disintegrin protein family, appears to be required for myotube formation. Involvement of a fertilin-related protein in myogenesis suggests that there are common mechanisms in gamete and myoblast fusion.