The present invention provides a method for the mutation, synthesis and selection of a protein which binds to a target molecule, the method comprising: (a) incubating a replicable mRNA molecule encoding the protein with ribonucleoside triphosphate precursors of RNA and an RNA-directed RNA polymerase, wherein the RNA-directed RNA polymerase replicates the mRNA molecule but introduces mutations thereby generating a population of mutant mRNA molecules; (b) incubating the mutant mRNA molecules from step (a) with a translation system under conditions which results in the synthesis of population of mutant proteins such that after translation, mutant proteins are linked to their encoding mRNA molecules thereby forming a population of mutant proteins such that after translation, mutant proteins are linked to their encoding mRNA molecules thereby forming a population of mutant/mRNA complexes; (c) selecting one or more mutant protein/mRNA complex(es) by exposing the population of mutant protein/mRNA complexes from step (b) to the target molecule and recovering the mutant protein/mRNA complex(es) bound thereto.