Orphan receptor

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Orphan receptors are apparent receptors that have a similar structure to other identified receptors but whose endogenous ligand have not yet been identified. If a ligand for an orphan receptor is later discovered, the receptor is referred as "adopted ophan receptor".

Examples of orphan receptors are found in the G protein-coupled receptor[1][2][3] and nuclear receptor[4] families. Examples of adopted orphan receptors in the nuclear receptor group include the liver X receptor (LXR), farnesoid X receptor (FXR), and peroxisome proliferator-activated receptor (PPAR).

Historically, receptors were discovered by using ligands to "fish" for their receptors. Hence by definition, these receptors were not orphans. However with modern molecular biology techniques such as screening of cDNA libraries, it becomes possible to identify related receptors based on sequence similarity to known receptors without knowing what their ligands are.


  1. Levoye A, Dam J, Ayoub MA, Guillaume JL, Jockers R (2006). "Do orphan G-protein-coupled receptors have ligand-independent functions? New insights from receptor heterodimers". EMBO Rep. 7 (11): 1094–8. doi:10.1038/sj.embor.7400838. PMID 17077864.
  2. Civelli O, Saito Y, Wang Z, Nothacker HP, Reinscheid RK (2006). "Orphan GPCRs and their ligands". Pharmacol Ther. 110 (3): 525–32. doi:10.1016/j.pharmthera.2005.10.001. PMID 16289308.
  3. Wise A, Jupe SC, Rees S (2004). "Orphan GPCRs and their ligands". Annu Rev Pharmacol Toxicol. 44 (February): 43–66. doi:10.1146/annurev.pharmtox.44.101802.121419. PMID 14744238.
  4. Benoit G, Cooney A, Giguere V, Ingraham H, Lazar M, Muscat G, Perlmann T, Renaud JP, Schwabe J, Sladek F, Tsai MJ, Laudet V (2006). "International Union of Pharmacology. LXVI. Orphan nuclear receptors". Pharmacol Rev. 58 (4): 798–836. doi:10.1124/pr.58.4.10. PMID 17132856.