We have previously shown that v-erb A expression strongly stimulates quail myoblast proliferation and di erentiation without alteration of the triiodothyronine (T3) in¯uence in this cell type. In order to understand the molecular basis of v-erb A action in myoblasts, we have studied the in¯uence of this oncoprotein on c-erb Aa1 encoded T3 nuclear receptor (TRa) activity. In transfection experiments, v-erb A did not inhibit the T3-dependent c-erb Aa1 transcriptional activity in QM7 myoblasts in contrast to its action in HeLa cells. However, it repressed the retinoic acid receptor RARa activity in both cell-types, indicating that v-erb A interactions with T3 or RA mediated transcription signi®cantly di ers. In EMSA experiments using a TRE pa1 probe, T3Ra binds as three complexes in HeLa cells. We have previously identi®ed the slow migrating complex, undetectable in QM7 myoblasts, as a T3R/ RXR heterodimer. Interestingly, v-erb A inhibited binding of this complex in HeLa cells, but did not a ect binding of the two other complexes in QM7 myoblasts. Expression of RXR (g isoform), the TRa dimerization partner absent in proliferating QM7 cells, restored inhibition of c-erb Aa1 transcriptional activity in these cells and abrogated the v-erb A myogenic in¯uence. Lastly, v-erb A induced a T3-independent cerb Aa1 activity in QM7 cells when cotransfected in equimolar ratio with the receptor, by inhibiting AP-1 activity and stimulating transcription of a reporter gene driven by a TRE sequence.