Immunohistochemical markers are used in various cancers. Her-2/Neu immunohistochemistry in breast cancer is used for:

HER2, also known as neu or c-erbB-2, is a member of the epidermal growth factor receptor (EGFR) family and is one of the best-characterized tyrosine kinases. The HER2 gene is frequently amplified and the protein is overexpressed in many cancers, including breast, ovarian, lung, gastric, and oral cancers. HER2 overexpression is associated with increased cell proliferation and anchorage-independent growth as well as resistance to proapoptotic stimuli. HER-2/neu amplification/ overexpression markers in breast cancer have been validated to predict response to therapy in breast cancer. Ref: Meric-Bernstam F., Pollock R.E. (2010). Chapter 10. Oncology. In F.C. Brunicardi, D.K. Andersen, T.R. Billiar, D.L. Dunn, J.G. Hunter, J.B. Matthews, R.E. Pollock (Eds), Schwaz's Principles of Surgery, 9e.