- ErbB2 overexpressing breast cancer, defined as 3+ staining by immunohistochemistry (IHC), or 2+ staining by IHC in conjunction with ErbB 2 gene amplification by FISH, or ErbB 2 gene amplification by FISH alone (in subjects whose tumor blocks were not assessed by IHC). 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;