Wiktionary
orphan receptor
n. (context physiology English) A receptor that has a similar structure to other identified receptors but whose endogenous ligand has not yet been identified
Wikipedia
Orphan receptor
An orphan receptor is an apparent receptor that has a similar structure to other identified receptors but whose endogenous ligand has not yet been identified. If a ligand for an orphan receptor is later discovered, the receptor is referred to as an "adopted orphan".