The single chain M13 filamentous phage display system. Filamentophages are a large family of viruses capable of infecting negative bacteria. They contain single chain DNA genomes and tubular capsid proteins, and usually consist of thousands of copies of main capsid protein (PⅧ) and the minor capsid proteins (PⅢ) located at the terminus. This M13 bacteriophage system is divided into four systems: PⅢ, PⅧ, PV and D protein.