A method of selectively introducing a substituent into a protein proximal to a binding site on the protein for a homing peptide, comprising: (a) contacting the protein with a compound comprising a homing peptide having the ability to bind to the binding site of the protein and (b) allowing a moiety on the protein proximal to the binding site to react with the compound comprising the homing peptide, thereby to transfer the substituent G onto the protein.
