Immunohistochemistry has been used in the past to identify overexpression of HER-2/neu protein on the cell membrane of breast cancer cells in fixed tissues. Most studies that have attempted to find an association between HER-2/neu expression and a poor prognosis have relied on this technique, which has intrinsic variability due to the antibody used and the degradation of surface proteins by fixation. Recent studies with fluorescence in situ hybridization have tended to confirm the purported association, showing that HER-2/neu overexpression causes a more aggressive, less responsive breast cancer. In many studies, the amplification of the HER-2/neu gene was the most important variable determining outcome, independent of other variables, such as tumor size and estrogen receptor status.