Nuclear receptors are members of a large superfamily of proteins that function as ligand-inducible transcription factors (Germain et al. 2006; Teboul et al. 2008). This family contains steroid hormone receptors such as the glucocorticoid, estrogen, and androgen receptors, as well as receptors for thyroid hormones and retinoic acid. In addition, orphan nuclear receptors exist for which ligands have not been identified (Teboul et al. 2008). Examples of such orphan receptors include HNF-4α and chicken ovalbumin upstream promoter-transcription factors (Benoit et al. 2006). These receptors regulate gene transcription by binding to hormone response elements in the promoter region of target genes. Most receptors bind as homo- or hetero-dimers to response elements composed of two core hexameric motifs. Consensus sequences for these motifs include AGAACA for steroid receptors and AG(G/T)TCA for the remaining nuclear receptors (Aranda & Pascual 2001). Multiple nuclear receptor types can bind these sequences and mediate transcriptional activity, allowing for differential control of overlapping gene networks (Bedo et al. 1989; Umesono et al. 1991). Nuclear receptors have a well established role in growth, development and homeostasis as has been reviewed (Flamant et al. 2006).
