ESR1 and breast cancer: In the study by Oh et al., to examine the risk factors of breast cancer with regard to estrogen receptor, progesterone receptor, insulin-like growth factor, and expression of Ki67 in normal breast tissue in 2017, the median level of estrogen expression was significantly higher in older women (equal to or greater than 50 years) than among those younger than 50 years of age, but the expression of progesterone had no significant relationship with age [18].