"Under physiological conditions, virtually all vitamin A in the blood is not in its free form but bound to its high-affinity carrier protein. If this carrier protein is like a boat that carries vitamin A, how does it know when to unload its cargo? It was proposed more than 30 years ago that a cell-surface receptor exists that specifically binds to the carrier protein and mediates vitamin A uptake."
Feel free to be the first
Please review the linked page for context.
If you can think of something better than this,
please add it to the database