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 and nuclear receptor 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.
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