However, immunohistochemical (IHC) analysis reveals that breast cancer is a heterogeneous disease as evidenced by the differential detection of two main targetable tumorigenic drivers: estrogen receptor alpha (ERα), a nuclear receptor, and ERBB2/HER2, a membrane receptor belonging to the ERBB1-4 growth factor receptor family whose gene is frequently amplified in breast cancer. Here, ESR1 is linked to breast cancer.