ESR1 and breast cancer: In preclinical mouse models our group has shown that treatment with gefitinib, an EGFR kinase inhibitor, delayed the development of spontaneous ER-negative, HER2-positive mammary tumors in the MMTV-ErbB2 transgenic mouse model [17] (median time to tumor formation in the control group 230 days versus 310 days in the high-dose gefitinib group, p < 0.001) (Table 1).