摘要: The nuclear receptor superfamily [1] consists of a class of transcription factors with a common DNA-binding structural motif (Fig. 1A). The first described members of this family were the receptor proteins for the five different classes of steroid hormones. It was subsequently shown that vitamin D, thyroid hormone, retinoids and fatty acids exert their biological effects with the help of similar receptor proteins that belong to the same superfamily. In addition to these liganddependent transcriptional factors, a large number of related proteins have been cloned, based on their structural homology to the DNA-binding motif. Some of these proteins have been shown to be activated by other mechanisms, such as phosphorylation, instead of binding of a specific ligand [2]. However, for the majority of these related proteins, the biological function and putative ligands are still not known. These related proteins are called the orphan receptors.