ESR1 and breast cancer: Apart from ERα gene (ESR1) mutations, endocrine resistance in advanced breast cancer is also associated with activating HER2 and loss of function NF1 mutations, alterations in other mitogen-activated protein kinase (MAPK) pathway genes including EGFR, KRAS, HRAS, BRAF, and MAP2K1 (MEK1), as well as ER transcriptional regulators such as MYC, CTCF, FOXA1, and TBX3 (44).