- OBJECTIVES: To evaluate fluorescence in situ hybridization for distinguishing between malignant and benign breast tumors and determining genetic subgroups of breast cancers. STUDY DESIGN: Touch preparations from 94 surgically removed breast tumors (17 benign and 77 malignant) were hybridized with a DNA probe specific for centromeric DNA sequences of chromosome 1. Twenty samples were additionally hybridized with a chromosome 9-specific probe. RESULTS: We investigated the heterogeneity of the cell populations on the basis of the number of signals per nucleus. All benign tumors showed two signals per nucleus. In contrast, carcinomas revealed a broad spectrum of hybridization patterns. Some showed almost exclusively two signals per nucleus, and others exceeded four signals. CONCLUSION: The hybridization patterns of individual tumors can be used for defining different subsets of breast cancer. The results may have prognostic impact, leading to "molecular-cytogenetic grading" of breast cancer.