Antiproliferative effects of erlotinib correlate poorly with abundance of p-EGFR. Antiproliferative effects of erlotinib correlate poorly with abundance of p-EGFR. The U87MG panel was treated with 3 doses of erlotinib, as shown, then pulsed with 100 ng/mL EGF before harvesting. Phospho- and total protein levels were visualized by Western blotting. Although low dosages of erlotinib efficiently blocked p-EGFR (Y1173) in all cell lines, levels of p-ERK 1/2 (T202/Y204) were decreased in NSCLC-derived alleles, compared with brain cancer–derived EGFRvIII, paralleling the antiproliferative response in Figure 1B. U87MG cells are mutant for PTEN, disconnecting signaling between EGFR and PI3K. Thus, the differential abundance of p-AKT (S473) was less apparent in this experiment, as compared with PTENWT LN229 cells (Supplemen-tary Fig. S10). Levels of kinase-site occupancy more closely follow abundance of phosphorylated downstream molecules. Krister J. Barkovich et al. Cancer Discovery 2012;2:450-457 ©2012 by American Association for Cancer Research