- Unequivocal ErbB2 overexpressing breast cancer, defined as 3+ staining by immunohistochemistry (IHC), or 2+ staining by IHC in conjunction with ErbB2 gene amplification by FISH/CISH, or ErbB2 gene amplification by FISH/CISH alone (in subjects whose tumor blocks were not assessed by IHC). ErbB2 gene amplification is defined by: > six (6) ErbB2 gene copies/nucleus for test systems without an internal control probe or an ErbB2/CEP 17 ratio of more than 2.2.