desirable level of mutational activity at this point the mutant gene carrying phagemid particles may be propagated into a suitable host cell or manipulated according to known methods to induce expression protein encoded by the mutant gene a preferred method is to combine the generation of mutations and the selection of favourable mutations into one process for that purpose the gene of interest is expressed in the mutator strain and variants which exhibit increased activity of the gene are being enriched by selection or screening infection or transfection of host cells with helper phage or phagemid particles in the phage state may be accomplished according to well known techniques in the art see e g sambrook and russel et al gene 1986 vol 45 pp 333 338 the helper phage which find utility in the claimed methods may be any phage which can be used in combination with the specific cloning phagemid used which is able to produce an infective transducing phagemid particle for example in one embodiment where the cloning vector may be a phagemid the helper phage may be ml3 or f1 by following the teachings of the invention provided herein the mutant gene bearing phagemid is separated.