ERBB2 and breast cancer: Recent studies classify breast cancer into four subtypes: Luminal A (ER+/PR+/HER2−, grade 1 or grade 2), Luminal B (ER+/PR+/HER2+, or ER+/PR+/HER2− grade 3), HER2 overexpression (ER−/PR−/HER2+), and triple negative breast cancer (TNBC, ER−/PR−/HER2−).