The invention relates to a method of directing selection of biologicaltherapeutic molecules to specific functional domains of the target biologicmolecule. Selection is directed by the use of closely related molecules, whereone is a decoy and the other contains the targeted domain or epitope. Theinvention is based on the use of physical data, which may be combined withderived data, to ascertain that the decoy and the target differ only in thespecific functional domain or epitope where the binding will be directed.