Importantly, for MAPK1p.D321N mutant tumors, erlotinib treatment resulted in a significant reduction in tumor volume (60.3% reduction vs. vehicle treatment, P = 0.0037; Fig. 2a), with concomitant increases in tumor-negative areas (i.e., cytokeratin-negative areas; Fig. 2c), as well as dramatic decreases in activated p-EGFR (membranous) in the tumors with <20% of membranous p-EGFR signals remaining (Fig. 2b). The gene discussed is EGFR; the disease is neoplasm.