HER2 overexpression in breast tumors. Representative IHC staining for HER2 expression in invasive breast carcinomas (A-D). HER2 is predominantly membrane-bound (B-D). (A) Tumor that is negative for HER2. (B) Tumor with moderate HER2 staining (HercepTest™ 2+ staining). (C,D) Tumor with strong HER staining (HercepTest™ 3+ staining). Magnification: 100× for C; 200× for A,B,D. Counterstain: Hematoxylin.