ESR1 and neoplasm: Most often, an immunohistochemical (IHC) study is used to determine the subtype of the disease, which allows determining the level of expression of receptors on the surface of tumor cells – estrogen receptors (ER), progesterone receptors (PR), and oncoprotein HER-2/neu (human epidermal growth factor receptor 2 – the second epidermal growth factor receptor).