ESR1 and breast cancer: Breast cancer is the most widespread female malignancy in the world and it is divided into three different subtypes by immunohistochemistry classification: estrogen/progesterone positive (ER+/PR+), human epidermal growth factor receptor 2 positive (HER2+) and triple negative breast cancer (TNBC); and in five different subtypes by molecular classification: Luminal A, Luminal B, HER2-enriched, basal-like breast cancer and normal-like [2].