While the proliferation of normal cells is strictly regulated, cancer cells exhibit aberrant regulation of cell proliferation.18 Cancer cells may overexpress either the receptors or ligands of growth factors, such as androgen receptor (AR) in prostate cancer and estrogen receptor (ER) in breast cancer.19,20 RTKs are also commonly deregulated in the cancer cells, including epidermal growth factor receptor (EGFR), human epidermal growth factor receptor 2 (HER2), and Bcr-Abl.21,22 In addition, aberrant activation of transcription factors are also important for carcinogenesis. Here, ESR1 is linked to breast cancer.