ESR1 and breast cancer: The majority of oestrogen receptor (ER)-positive and human epidermal receptor 2 (HER2)-negative (ER+/HER2–) breast cancers are classified as Luminal A or Luminal B. An important difference between these groups is that patients with Luminal B tumours have a higher risk of relapse [2] and are therefore often recommended adjuvant chemotherapy in addition to endocrine therapy [3].