ESR1 and breast cancer: One recent in vitro study demonstrated that tamoxifen- and fulvestrant-resistant breast cancer cell lines were sensitive to treatment with lapatinib or afatinib (a second-generation anilinoquinazoline that irreversibly binds to the intracellular tyrosine kinase domains of EGFR, HER2, and HER4 receptors), and gradual reactivation of ERα sensitivity was observed with lapatinib therapy34.