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A protein with a receptor structure but with unidentified ligand
In biochemistry, an orphan receptor is a protein that has a similar structure to other identified receptors but whose endogenousligand has not yet been identified. If a ligand for an orphan receptor is later discovered, the receptor is referred to as an "adopted orphan". Conversely, the term orphan ligand refers to a biological ligand whose cognate receptor has not yet been identified.
GPCR orphan receptors are usually given the name "GPR" followed by a number, for example GPR1. In the GPCR family, nearly 100 receptor-like genes remain orphans.
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 and whole genome sequencing, receptors have been identified based on sequence similarity to known receptors, without knowing what their ligands are.
^Mi LZ, Devarakonda S, Harp JM, Han Q, Pellicciari R, Willson TM, Khorasanizadeh S, Rastinejad F (April 2003). "Structural basis for bile acid binding and activation of the nuclear receptor FXR". Mol. Cell. 11 (4): 1093–100. doi:10.1016/S1097-2765(03)00112-6. PMID12718893.
^Fagg GE (May 1987). "Phencyclidine and related drugs bind to the activated N-methyl-D-aspartate receptor-channel complex in rat brain membranes". Neurosci. Lett. 76 (2): 221–7. doi:10.1016/0304-3940(87)90719-1. PMID2438606.