ESR1 and breast cancer: The binding of estrogen to the ESR1 is not only a key regulator for the physiological growth and differentiation of the mammary gland, it is also a key element in the malignant progression of breast cancer, i.e. the growth of ESR1 expressed breast cancer cells is stimulated by estrogen, which in turn makes it accessible to endocrine treatment strategies, while breast cancers that do not express ESR1 exhibit a primary resistance to endocrine treatment [3,4].