ESR1 and breast cancer: In particular 80–90% of ER+ breast cancer also express AR which associates with a more favourable prognosis.12,13 Nonetheless, pre-clinical studies indicate increased abundance of AR reduces response to both tamoxifen and oestrogen-deprivation14,15 an observation that has been supported in retrospective clinical investigations.14,16 Furthermore, pre-clinical studies have suggested the potential usefulness of therapies that target AR signalling in breast cancer16 and have paved the way for a number of clinical trials.17–19