When used as secondary detection reagent on BT-474 breast cancer cells, which strongly overexpress HER2, the labelled GFP-clamps resulted in a specific signal, very similar to that of a commercially available rat monoclonal antibody (FM264G) (Fig. 6(a)), which, however, may crosslink GFP and thus also the bound receptor and may thus elicit unintended biological consequences. The gene discussed is ERBB2; the disease is breast carcinoma.