Clinically, HER2+/ER+ tend to have a worse prognosis than HER2+/ER- patients, therefore, in this study we aimed to assess differences found within the HER2 subtype by characterizing what impact the presence or absence of the ER may have on the bivalent promoters within the HER2+ breast cancer cell lines. The gene discussed is ESR1; the disease is breast carcinoma.