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Basal-like breast cancer (BLBC) is an aggressive and deadly subtype of human breast cancer that is highly metastatic, displays stem-cell like features, and has limited treatment options. Therefore, developing and characterizing preclinical mouse models with tumors that resemble BLBC is important for human therapeutic development. <i>ATF3</i> is a potent oncogene that is aberrantly expressed in most human breast cancers. In the BK5.ATF3 mouse model, overexpression of <i>ATF3</i> in the basal epithelial cells of the mammary gland produces tumors that are characterized by activation of the Wnt/β-catenin signaling pathway. Here, we used RNA-Seq and microRNA (miRNA) microarrays to better define the molecular features of BK5.ATF3-derived mammary tumors. These analyses showed that these tumors share many characteristics of human BLBC including reduced expression of <i>Rb1</i>, <i>Esr1</i>, and <i>Pgr</i> and increased expression of <i>Erbb2</i>, <i>Egfr</i>, and the genes encoding keratins 5, 6, and 17. An analysis of miRNA expression revealed reduced levels of <i>Mir145</i> and <i>Mir143</i>, leading to the upregulation of their target genes including both the pluripotency factors <i>Klf4</i> and <i>Sox2</i> as well as the cancer stem-cell-related gene <i>Kras</i>. Finally, we show through knock-down experiments that <i>ATF3</i> may directly modulate <i>MIR145/143</i> expression. Taken together, our results indicate that the <i>ATF3</i> mouse mammary tumor model could provide a powerful model to define the molecular mechanisms leading to BLBC, identify the factors that contribute to its aggressiveness, and, ultimately, discover specific genes and gene networks for therapeutic targeting.