Several investigations have supposed that tumor suppressor genes might be located on human chromosome 8. We used microcell-mediated transfer of chromosome 8 into MDA-MB-231 breast cancer cells and generated independent hybrids with strongly reduced tumorigenic potential. Loss of the transferred chromosome results in reappearance of the malignant phenotype. Expression analysis identified a set of 109 genes (CT8-ps) differentially expressed in microcell hybrids as compared to the tumorigenic MDA-MB-231 and rerevertant cells. Of these, 44.9% are differentially expressed in human breast tumors. The expression pattern of CT8-ps was associated with prognostic factors such as tumor size and grading as well as loss of heterozygosity at the short arm of chromosome 8. We identified CT8-ps networks suggesting that these genes act cooperatively to cause reversion of tumorigenicity in MDA-MB-231 cells. Our findings provide a conceptual basis and experimental system to identify and evaluate genes and gene networks involved in the development and/or progression of breast cancer.