ESR1 and breast cancer: These subtypes are based on the hormone receptor status expressed on the breast cancer cells (estrogen (ER) and/or progesterone (PR) receptors), the amplification/overexpression of the epidermal growth factors receptor 2 (HER2/Neu) with or without ER/PR expression, and the breast cancer cells not expressing any receptors/proteins classified as triple-negative breast cancer (TNBC) [3].