The invention relates to a method for artificial in vivo evolution of proteins, said method making it possible to bring about the evolution of a protein X by complementation of a relative protein Y, X and Y both belonging to the same class of enzyme commission (EC) nomenclature or belonging to related classes. The mutants D133E and R104Q of desoxycytidine kinase (DCK) were obtained; both of said mutations result in acquisition of thymidine kinase activity by DCK.