Gefitinib and erlotinib are specific inhibitors of EGFR,26 whereas CI-1033, in addition to blocking EGFR, also inhibits the kinase activity of Erbb2 and Erbb4 (Erbb3 does not possess intrinsic kinase activity).27 All treated cohorts of RT2 mice displayed a striking 50% to 60% decrease in tumor burden (average total tumor volume per mouse) compared to vehicle-treated cohorts (Figs. 1 D-F). Here, ERBB4 is linked to neoplasm.