THE thyroid-hormone receptors are hormone-dependent transcription factors that control expression of many target genes1,2. This regulation is presumably a consequence of hormone-dependent contacts between the receptors and the basal transcription machinery3. We used the yeast two-hybrid system 4,5 to identify a candidate human transcriptional mediator that interacts with both the thyroid-hormone receptor and the retinoid-X receptor in a ligand-dependent fashion. This protein, Tripl (for thyroid-hormone-receptor interacting protein), shares striking sequence conservation with the yeast transcriptional mediator Sugl (refs 6, 7). Here we show that Tripl can functionally substitute for Sugl in yeast, and that both proteins interact in vitro with the thyroid-hormone receptor, and with the transcriptional activation domains of yeast GAL4 and of herpes virus VP16.
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