The mrnadisplay technology enables the in vitroselection and directed evolution of functional proteins from libraries of more than 1012different mutants in a single test tube. the size of these libraries is well beyond the limit of screenin g technologiesand of most in vivoand in vitroselection methods. the mrnadisplay technology has been used to select peptides and proteins that bind to a specific ligand, as well as novel enzymes. this protocol details the procedure to produce mrna-displayed proteins (3 d) and to subject them to a selection and evolution of enzymes for bond-forming reactions (4-10 weeks). this method is demonstrated by the generation of new rnaligase enzymes.