PⅧ, the main coat protein of filamentous phages, lies in the lateral side of phages. Its C-terminus binds to DNA, with its N-terminus reaching out of phages. Each of the virus particles contains approximately 2,700 copies of PⅧ, in the vicinity of whose N-terminus pentapeptide can be fused. However, it cannot fuse longer peptides, because larger peptides or proteins may give rise to space obstacles, thereby affecting the phage assembly by making it lose infectivity. When helper phages are involved, the wild type of PⅧ protein can fuse polypeptide or even the antibody fragments.