Since early 1980s, adjuvant endocrine treatment involving the use of tamoxifen has shown to reduce recurrence and increase survival in estrogen receptor (ER) positive breast cancer patients (Refs 1, 2) by preventing tumour cell growth and angiogenesis, potentially via impeding the binding of estrogen to the ER or inhibiting the expression of estrogen-responsive genes (Ref. This evidence concerns the gene ESR1 and breast cancer.