Amplification of the c-erbB-2 (HER-2/neu) proto-oncogene is detected in 10-30% of human breast cancers and has been shown to be accompanied by the overexpression of its protein in the cancer cell membrane. c-erbB-2 gene amplification is one of the first genetic alterations to be used clinically as a prognostic indicator, a predictive factor of response to doxorubicin (adriamycin) chemotherapy, and a test of patient eligibility for therapy with trastuzumab, a humanized anti-c-erbB-2 antibody. There are two types of tests to detect c-erbB-2 amplification/overexpression: immunohistochemistry and fluorescence in situ hybridization (FISH). Accurate identification of cases with c-erbB-2 amplification/overexpression requires an optimized combination of immunohistochemical and FISH tests.