With this chemistry, fd phage bearing single-chain antibody fragment (scFv) groups previously determined to bind epidermal growth factor receptor (EGFR; Heitner et al., 2001) and human EGFR 2 (HER2; Poul et al., 2000) were modified with fluorophores and polyethylene glycol 2k (PEG2k) to create phage constructs that could distinguish breast cancer cell markers using fluorescence microscopy. The gene discussed is EGFR; the disease is breast cancer.