To molecularly characterize mouse mammary gland tumors and address the possible importance of transcriptional dysregulation in the BK5.ATF3 tumor model, we examined both mRNA and miRNA expression patterns in mammary tumor tissue derived from parous BK5.ATF3 female mice compared to the expression patterns in normal, adjacent mammary glands of the same transgenic mice. The gene discussed is ATF3; the disease is breast cancer.