ESR1 and breast cancer: Breast cancer has surpassed lung cancer in terms of new cases, becoming the most extensively diagnosed cancer in 2020 and the fifth chief cause of cancer death.[1] Nearly 70% of breast cancer patients are estrogen receptor‐positive (ER+), making them candidates for endocrine therapies such as tamoxifen, fulvestrant, and letrozole.[2] Tamoxifen, a selective ER modulator, binds to ER competitively and inhibits ER‐induced breast cancer cell growth.