A polymer comprising i) a synthetic polymeric component that has been formed from an ethylenically unsaturated water-soluble or potentially water-soluble monomer and an ethylenically unsaturated monomer carrying a reactive group, wherein the reactive group is directly reactive with a cellulose binding domain protein; and ii) a protein component consisting of a cellulose binding domain protein, wherein the cellulose binding domain protein. The polymer is particularly suitable as a dry strength resin in a paper making process. It can also be used as a wet strength resin, internal sizing agent or surface sizing agent.