Orphan receptor

In biochemistry, an orphan receptor is a protein that is structurally similar to a receptor, but for which no physiological relevant endogenousligand is known. If a biologically significant 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. Because it can take time for terminology in common use to change, and disagreements can exist about whether a discovered endogenous ligand has a biologically significant function, specific adopted orphans may continue to be referred to as orphans for some time after their ligand is found, despite no longer being true orphans.