A protein with a receptor structure but with unidentified ligand
This article needs to beupdated. The reason given is: Ligands are now partially identified along with a few other new discoveries. Please help update this article to reflect recent events or newly available information.(March 2024)
Inbiochemistry, anorphan receptor is a protein that has a similar structure to other identifiedreceptors but whoseendogenousligand has not yet been identified. If a ligand for an orphan receptor is later discovered, the receptor is referred to as an "adopted orphan".[1] Conversely, the termorphan 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 exampleGPR21. In the GPCR family, nearly 100 receptor-like genes remain orphans.[11]
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 asreverse pharmacology, screening ofcDNA libraries, andwhole genome sequencing, receptors have been identified based on sequence similarity to known receptors, without knowing what their ligands are.
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