BC is a genetically and morphologically highly heterogeneous disease, which is classified based on the immunohistochemical expression of hormone receptors into subtypes: ER+, progesterone-receptor-positive (PR+), human-epidermal-growth-factor-receptor-positive (HER2+), and triple-negative breast cancer (TNBC) with none of the above-mentioned receptors [169]. Here, ESR1 is linked to breast cancer.