ESR1 and cancer: Breast cancer has become the leading cause of cancer incidence worldwide, and the incidence and mortality rate of breast cancer were the highest in most countries among women in 2020.[1] Triple‐negative breast cancer (TNBC), lacking the expression of estrogen receptor (ERα), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), accounts for 15%‐20% of breast cancer incidence.