ESR1 and breast neoplasm: Thus, hypersensitivity to E2 and subsequent ER‐mediated cell death may be enhanced in tumors harboring genomic amplification of ESR1. ESR1 amplification has been observed in up to 20% of metastatic ER+ breast tumors from patients (Holst, 2016; Lefebvre et al., 2016; Lin et al., 2013).