The purpose of the present invention is to provide a novel protein involved in adhesion and fusion between myoblasts in the course of the formation of byotube. The present invention relates to Meltrins, which a membrane protein having fusion, adhesion and aggregation activity of cells, especially myoblast; and to polypeptides of their domains, DNAs encoding them, antisense RNA for these DNAs, various antibodies to Meltrins and the polypeptides of their domains, expression vectors containing these DNAs, and transformants by these vectors; as well as to the process for producing Meltrins and the polypeptides of their domains using those transformants and medical compositions comprising Meltrins or antagonist against them as an effective ingredient.