A method to identify a peptide and/or its encoding DNA, wherein the peptide binds to a T-cell receptor and/or an NK-cell receptor and/or an NKT-cell receptor, the method comprising: providing a carrier to which is attached a peptide and its encoding DNA; providing an MHC and/or an MHC-like molecule; and exposing the peptide to a T-cell receptor and/or an NK-cell receptor and/or an NKT-cell receptor in the presence of the MHC or MHC-like molecule.