ESR1 and breast cancer: Hormone‐responsive breast cancers are also receptive to FGF signalling, where studies have shown that FGF overexpression can promote oestrogen‐independent proliferation in ER+ breast cancers,65 and that signalling by FGFR2 may confer resistance to tamoxifen through degradation of ER,56, 66 or even that oestrogen may mediate FGF paracrine activation.67