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Transcriptional activation (TA) mediated by the effect of thyroid hormones on target genes requires co-activator proteins such as the early region 1A (E1A) associated 300 kDa binding protein (p300) and the cAMP response element binding protein (CREB) binding protein (CBP), known as the p300/CBP complex, which acetylate histones 3 and 4 to allow transcriptional machinery access to the target gene promoter. Little is known on the role of p300 in thyroid hormone receptor (TR) mediated TA but the E1A-like inhibitor of differentiation 1 (EID1), an inhibitor of p300 histone acetyltransferase (HAT), is a functional homolog of E1A and may inhibit myogenic differentiation factor D (MyoD) transcriptional activity and reduces muscle cell differentiation. We evaluated the influence of EID1 on TR-mediated transcriptional activity using transfection and mammalian two-hybrid studies to show that EID1 may partially reduces TA activity of the TR receptor, probably due to p300 blockage since EID1 mutants cannot reduce TR-mediated TA. The EID1 does not affect the function of p160 co-activator proteins (160 kDa proteins of steroid receptor co-activators) and is functionally independent of co-repressor proteins or TR binding. Summarizing, EID1 reduces TR-mediated transcriptional activity by blocking p300 and may play an important role in thyroid receptor activity in muscle and other tissues.