Further highlighting how intratumoral heterogeneity may delineate clinical responses to HER2-targeted strategies, the authors observed a greater reduction in tumor burden to afatinib in cases where EGFR and HER2 gene co-amplification occurred, interestingly in cases where the co-amplification existed within the same clonal tumor cell population confirmed by dual-probe FISH. The gene discussed is ERBB2; the disease is neoplasm.