EGFR and HER2 increase at the plasma-membrane level. Calu-3 (A) and H292 (B) cell lines were treated with 1 μM erlotinib for 24 h, H322 cell line was treated with increasing concentration of erlotinib (C) or with 1 μM erlotinib for the indicated period of time (D). At the end of the treatment, cell surface expression of EGFR and/or HER2 were evaluated by flow cytometry and the quantification is reported as Molecules of Equivalent Fluorophore [MEF] or as fold increase versus untreated control cells (D). Inset Figure 2A: Western blot analysis of EGFR protein membrane level in Calu-3 after treatment with 1 μM erlotinib for 24 h. Whole cells were labeled with biotin and membrane bound proteins were pulled down with neutrAvidin beads. The results are from representative experiments. Each experiment, repeated three times, yielded similar results.