Oncogene amplification in breast cancer. To refine the analysis of gene amplification in breast cancer, the authors have developed sensitive methods that can be used to screen nucleic acid prepared from a variety of sources. In their analysis, Southern hybridization and DNA dot-blot analysis were used to screen 49 breast cancer DNAs for Myc, Neu, and Int-2 gene amplification. The analysis detected minimal one extra gene copy) as well as expanded (two or more extra gene copies) gene amplifications, and in addition, distinguished between gene amplification and aneuploidy as the cause of extra gene copies. These quantitative methods were adapted to patient specimens routinely available in the anatomic pathology laboratory, including fresh tumor tissue, tumor nuclei discarded during estrogen receptor analysis, and paraffin blocks. One minimal gene amplification was found in three cases of intraductal cancer. Of 25 cases of nonmetastatic invasive cancer, 28% had at least one extra Myc gene, whereas 24% had Neu, and 21% had Int-2 gene amplification. Of 21 cases of metastatic invasive cancer, 43% had Myc, 43% had Neu, and 40% had Int-2 gene amplification. Among the nonmetastatic cancers, 47% had one, 12% had two, and 4% had three amplified genes. Within the metastatic cancers, 48% had one, 28% had two, and 5% had three amplified genes. Our data suggest relationships between tumor progression and both incidence and size of Myc, Neu, and Int-2 gene amplification. 