ESR1 and cancer: Presently, the treatment approach for hormone-dependent breast tumors is to block the action of E2 on cancer cells via one of three approaches: (a) preventing E2 from associating with ER with an anti-E2, for example tamoxifen [64,65]; (b) inhibiting E2 synthesis with an aromatase inhibitor [66]; or (c) downregulation of ER protein levels using an anti-E2, for example fulvestrant (faslodex/ICI 182,780) [69].