ESR1 and breast cancer: While the presence of ERα promises a more favourable 5-year outcome compared to ERα-negative breast cancers, ERα-positive patients have a greater long-term risk of fatal disease as a consequence of de novo and acquired resistance to conventional anti-oestrogen therapy, and disease recurrence, even several decades after the primary diagnosis (Pan H. et al., 2017; Lindström et al., 2018).