ERBB2 and neoplasm: The most commonly used methods to determine the HER2 and HER3 status, in formalin-fixed paraffin-embedded tissue, are (a) immunohistochemistry (IHC), which measures the number of HER2 and HER3 receptors on the cell surface and therefore detects receptor overexpression and (b) fluorescence or chromogenic in situ hybridization (FISH and CISH, respectively), which detects gene amplification by measuring the number of copies of the HER2 and HER3 gene in the nuclei of tumor cells.