Moreover, BC can also be grouped according to immunohistochemical markers (ICM), like (i) presence or absence of receptors like those for estrogen (ER), progesterone (PR), human epidermal growth factor receptor-2 (HER-2) or epidermal growth factor receptor (EGFR) on tumor cell surface; (ii) expression of nuclear protein Ki67 as a marker of cell proliferation; and (iii) cytokeratin 5 expression in the plasma of BC cells. This evidence concerns the gene ESR1 and breast cancer.