Consequently, four biological markers assessed by immunohistochemistry (IHC), i.e. estrogen receptor (ER), progesterone receptor (PgR), HER2 and the cell proliferation marker Ki67 can classify breast cancer in five subtypes: luminal A (ER-positive and/or PgR-positive, HER2-negative and Ki67low), luminal B (ER-positive and/or PgR-positive, HER2-negative and Ki67high), luminal-HER2 (ER-positive and/or PgR-positive and HER2-positive), HER2-enriched (ER-negative, PgR-negative, HER2-positive) and triple-negative (ER-negative, PgR-negative, HER2-negative) [24]. Here, ESR1 is linked to breast cancer.