Breast cancer is the most common cancer and the second most frequent cause of cancer death in women. Epidemiological data has recognized that an increased cumulative exposure to estrogen is the common tie linking most of the established risk factors for breast cancer. Sex hormone-induced mammary gland carcinogenesis of the Noble rat (using testosterone and 17beta-estradiol) resembles that of the human counterpart in its growth pattern as well as the histopathology of the tumors induced. This model may provide a paradigm for examination of genetic alterations and changes in gene expression between different histological groups and to make inferences about the role of known and putative oncogenes and tumor suppressor genes. We studied the gene expression profile during sex hormone-induced mammary carcinogenesis using a cDNA array technique; the results were further confirmed by RT-PCR, western blotting and immunohistochemical analyses. From the 10 differentially expressed genes identified, we have studied four highly overexpressed genes, two cell cycle/growth control regulators, the cyclins D1 and D2, a growth factor, IGF-2 and a cytokine TNF-alpha. Cyclins D1 and D2 were highly expressed in the nuclei of carcinoma cells but at low levels in the nuclei of the hyperplastic and normal mammary tissue. IGF-2 was found to expressed in the cytoplasm of the carcinoma cells but not in the stromal cells. Western blot showed expression of big IGF-2 consistent with the tumor derived truncated forms of pro-IGF-2. The matured circulating IGF-2 at 7.5 kDa identified in the serum was not expressed in any of the breast tissue samples. TNF-alpha expression was found not only in the macrophages but also in the mammary carcinoma cells. The result of the present study provides some information on the molecular basis of this sex hormone-induced mammary carcinogenesis and the role of these proteins in tumor progression.