We have previously established that the antiproliferative protein BTG1 is a T3 target involved in the myogenic influence of the hormone. Inhibition of AP-1 activity by the liganded TR at the onset of myoblast terminal differentiation induces BTG1 expression. Furthermore, the nuclear localization of this protein is potentiated by T3. Lastly, BTG1 overexpression mimicks the myogenic influence of T3 by accelerating myoblast withdrawal from the cell cycle and stimulating their terminal differentiation. In search for the molecular basis of this myogenic influence, we established in transient transfection assays that BTG1 coexpression stimulates TR alpha and beta and myogenic factors (Myf5, MyoD, Myogenin) transcriptional activity. Moreover, using GST-pull down and coimmunoprecipitation experiments, we demonstrated that BTG1 directly interacts with these transcription factors. After identification of the sequences involved in these interactions, we constructed BTG1 mutants devoid of the interacting sequences with TRs or myogenic factors. We found that these interaction-defective mutants do not stimulate the transcriptional activity of MyoD or TRs, thus establishing that BTG1 is a common coactivator for TRs and myogenic factors. More interesting was the result indicating that, in contrast to wild-type BTG1, these mutants are devoid of any myogenic activity. These data clearly indicate that the ability of BTG1 to accelerate myoblast withdrawal from the cell cycle and to stimulate differentiation is essentially due to its coactivator function. Lastly, expression of a BTG1 antisens in myoblasts, strongly inhibited differentiation, suggesting that BTG1 is a major myogenic regulator. In conclusion, BTG1 is a new target of T3, acting as a common coactivator for TRs and myogenic factors, playing a major role in the regulation by the hormone of myoblast transition from proliferation to differentiation. As BTG1 is expressed in other cell types in which T3 regulates differentiation, this mechanism could be extended to other tissues