- ErbB2 overexpressing breast cancer, defined as 3+ staining by immunohistochemistry (IHC), or 2+ staining by IHC in conjunction with ErbB2 gene amplification by FISH, or ErbB2 gene amplification by FISH alone (in subjects whose tumor blocks were not assessed by IHC). ErbB2 gene amplification is defined by: > 6 ErbB2 gene copies/nucleus for test systems without an internal control probe or an ErbB2/CEP 17 ratio of more than 2.2. Subjects with tumors that are 2+ by IHC but negative or borderline by FISH assay are ineligible. For subjects with a history of more than one primary breast cancer, each breast cancer must be ErbB2 overexpressing to be eligible;