Breast cancer is the second leading cause of cancer death among women in the United States. The molecular mechanism of how the disease progresses is still unclear. Using N-nitroso-N-methylurea (NMU) induced rat mammary carcinogenesis as a model, we tried to gain a better understanding of the molecular events associated with initiation, promotion and progression stages of mammary carcinogenesis. To identify the biochemical pathways that regulate the early stage of carcinogenesis, we generated and compared the gene expression profiles of mammary cells after NMU exposure, in the presence and absence of dietary selenium, which has been shown to inhibit mammary carcinogenesis at the post-initiation stage. Interestingly, our results showed that a group of circadian genes were differentially regulated by selenium. Further analysis on the gene expression level of Per2 and DBP over a 24-hour period showed that, in the control mammary gland, the mRNA level of the circadian genes did not oscillate and selenium dramatically upregulated these genes when the dark cycle was initiated. The tight regulation of circadian gene expression implies the importance of maintaining a steady level of circadian gene expression in the hormone responsive mammary tissues, and it also suggests that the deregulation of the circadian genes may lead to or gain selective advantages to proceed to tumorigenesis as indicated by the downregulation of Per2 and DBP in mammary carcinomas. We also noted that mammary carcinomas from rats on the selenium diet, although not statistically different from control diet in percentage of tumors harboring H-ras mutation, tended to have higher level of activated H-ras allele in tumors with H-ras mutation. This led us to the study of H- ras gene regulation, which involved hnRNP A/B, a previously identified transcription factor that binds to the human and rat H-ras promoter with high affinity. A conditional knockout mouse model was generated to characterize the function of the gene. Our preliminary results showed that hnRNP A/B is not required for survival and development of the mice. The characterization of its role in carcinogenesis is currently under investigation. Together, our findings broaden the understanding of how mammary tumors progress at the molecular level.