ERBB2 and breast neoplasm: In addition, breast tumors can be classified into subtypes based on their expression profile, and each subtype is associated with distinct histological markers and clinical parameters: luminal A (estrogen receptor alpha (ER)- and/or progesterone receptor (PgR)-positive, HER2-negative), luminal B (ER- and/or PgR-positive, HER2-positive), HER2 (ER- and PgR-negative, HER2-positive), basal-like (ER-, PgR-, and HER2-negative; cytokeratin 5- and cytokeratin 6 (CK5/6)-positive and/or epidermal growth factor receptor (EGFR)-positive), and unclassified (negative for all five markers)[2,3].