COUP-TFs are orphan members of the steroid/thyroid hormone receptor superfamily. They are transcription factors that are highly conserved across species and have a wide spectrum of binding specificity. In tissue cultures, COUP-TFs repress the transactivation function of several other members of the steroid/thyroid hormone receptor superfamily, and they also negatively regulate the transcription from promoters of many other genes. The expression profile of COUP-TFs in several species suggests that they play an important role in development and differentiation.