ESR1 and breast cancer: For further analysis, using the ER and HER‐2 status of the tumor, breast cancer subtypes were approximated as follows: “luminal” (ER‐positive and HER‐2‐negative), “luminal HER‐2” (ER‐positive and HER‐2‐positive), “HER‐2” (ER‐negative and HER‐2‐positive), and “TNBC” (ER‐negative and HER‐2‐negative).