The NMDA receptor is a massive multi-subunit complex. CSHL researchers found that it looks much like a hot air balloon. The upper, balloon-like portion of the structure is found outside the cell and responds to chemical messengers. Those messengers act like a key to unlock the lower portion of the receptor. This lower portion, corresponding to the basket of the hot air balloon, is embedded in the neuron's membrane. It creates a narrow channel that allows ions, or electrically charged atoms, to flow into the cell. These many subunit interactions are potential targets for drug discovery.