ESR1 and breast cancer: Limitation of this study includes the following: limited total subjects (the limitation was caused by subject recruitment timeframe from August 2019 to October 2019), uneven distribution of each breast cancer stage subject representation (1A, 1B, 2A, and 2B) with only one subject included in 1B group absence of breast cancer subtype data, absence of BG estrogen receptor status, absence of serum estrogen level data, hormonal-affecting medication data, contraception data, and mixed subject history of curative surgery.