The study of proto-oncogene Her-2/neu using the fluorescence in situ hybridization (FISH) technique in routinely paraffin-embedded formalin-fixed tissue has become commonplace over the past decade and mandatory among invasive breast cancer expressing a score 2+ by immunohistochemical analysis of c-erbB2 protein. The patient's eligibility for treatment with the biological drug trastuzumab/herceptin is based on the evidence of a Her-2/neu proto-oncogene amplification (ratio Her-2/neu/CEP-17>2.2). However, although the exclusion is declared in the absence of Her-2/neu gene amplification (ratio Her-2/neu/CEP-17 <1.8) according to the American Society of Clinical Oncology/College of American Pathologists recommendations, there are borderline cases (1.8<ratio Her-2/neu/CEP-17>2.2) that need to be investigated (eg, ductal carcinoma in situ with microinvasion, metastatic breast cancer). In such cases with Her-2/neu genetic heterogeneity it is difficult to count the nuclear signals in the areas of invasive tumor using fluorescence. The availability of a Fluorescence Immunophenotyping and Interphase Cytogenetics as a Tool for Investigation of Neoplasms technique, based on the simultaneous evaluation of immunostaining with anticytokeratins (CKAE1/AE3 and CK19), together with FISH for Her-2/neu gene status [it is therefore useful and of current applicability in breast cancer blocks (formalin-fixed and paraffin-embedded)], permits a more easy identification of even single neoplastic cells by immunofluorescence and then a better evaluation of Her-2/neu status gene by the FISH technique, as shown in our study.