- Tumors must be HER2 overexpressing; acceptable methods of measurement of HER2 expression include immunohistochemistry IHC) and fluorescence in situ hybridization (FISH); tumors tested by IHC must be 3+ positive for HER2 overexpression; tumors tested by FISH must be positive by the specific FISH assay for genetic amplification of HER2; tumors that are 3+ by IHC, but negative by FISH assay are ineligible; consideration should be given to performing a repeat biopsy, with reanalysis for HER2 overexpression, in patients who have received prior trastuzumab, as little data exist on the persistence of HER2 overexpression after prior treatment with trastuzumab; biopsy in this circumstance, however, is not required