ESR1 and breast cancer: In atamoxifen-resistant, HER2-overexpressingMCF-7 breast cancer cell line, designated MCF-7/HER2-18, Gefitinib pretreatmentwas shown to block ER : EGFR receptor cross-talk, reestablish corepressorcomplexes with tamoxifen-bound ER on target gene promoters, eliminate tamoxifenagonistic effects, and restore tamoxifen antitumor activity both in vitro and in vivo [23].