We hypothesized that specific activation of a single retinoic acid receptor-α (RARα), without direct and concurrent activation of RARβ and γ, will inhibit mammary tumor oncogenesis in murine models relevant to human cancer. A total of 50 uniparous mouse mammary tumor virus (MMTV)-neu and 50 nuliparous MMTV-wnt1 transgenic mice were treated with RARα agonist (retinobenzoic acid, Am580) that was added to the diet for 40 (neu) and 35 weeks (wnt1), respectively. Among the shared antitumor effects was the inhibition of epithelial hyperplasia, a significant increase (P<0.05) in tumor-free survival and a reduction in tumor incidence and in the growth of established tumors. In both models, the mechanisms responsible for these effects involved inhibition of proliferation and survival pathways, and induction of apoptosis. The treatment was more effective in the MMTV-wnt1 model in which Am580 also induced differentiation, in both in vivo and threedimensional (3D) cultures. In these tumors Am580 inhibited the wnt pathway, measured by loss of nuclear b-catenin, suggesting partial oncogene dependence of therapy. Am580 treatment increased RARβ and lowered the level of RARγ, an isotype whose expression we linked with tumor proliferation. The anticancer effect of RARα, together with the newly discovered pro-proliferative role of RARγ, suggests that specific activation of RARα and inhibition of RARc might be effective in breast cancer therapy.