<p>The object is to construct a DNA replication reaction system which is excellent in general versatility and is easy to use. Disclosed is a monomer of PCNA which is a factor involved in DNA replication. The monomer has such an amino acid sequence that, when the monomer is linked to other monomer to form a polymer in which the N-terminus of the monomer and the C-terminus of the other monomer forms the boundary between these monomers, the amino acid residues of these monomers at the site at which an intermolecular interaction between these monomers is formed at the interface are those amino acids which repel each other by virtue of their electrical charges.</p>
