ESR1 and breast cancer: Pathologically, breast cancer is classified into breast invasive carcinoma (70–75%) and lobular carcinoma (12–15%) as suggested by the World Health Organization classification.102 There are also eighteen other uncommon subtypes, with a proportion of 0.5–5%.102 The pathological descriptions should also include the histological type, histological grade, immunohistochemistry assessment of hormone receptor (HR) status [estrogen receptor (ER) and progesterone receptor (PR) status], human epidermal growth factor receptor-2 (HER2) expression or HER2 gene amplification, and Ki67.