Provided is a method for the mutation, synthesis and selection of a protein of interest, by first incubating a replicable RNA molecule encoding the protein with ribonucleoside triphosphate precursors of RNA and an RNA-directed RNA polymerase, such that the RNA-directed RNA polymerase replicates the RNA molecule but introduces mutations thereby generating a population of mutant RNA molecules. The mutant RNA molecules are then incubated with a translation system under conditions which result in the synthesis of a population of mutant proteins. After translation, the mutant proteins are linked to their encoding RNA molecules, and one or more mutant proteins of interest are selected.