Robert Lefkowitz: Receptors are molecules, we now know, on cells, with which hormones and drugs interact to begin their biological actions. To give you a specific example, consider adrenaline, also known as epinephrine. Let’s say we have a patient with asthma and their airways are constricted. They can’t breathe. We give them adrenaline and the airways relax because of the smooth muscle in their airways relaxes when the adrenaline works on it. How does the adrenaline know to work on that, and to stimulate the heart, rather than to work on your nose or your retina or something like that? Well, the answer is, and what seemed obvious to me, is there must be molecules on the cells that the adrenaline would bind to, much like a key interacts with a lock, where the key would be the adrenaline, but it could be any hormone by extension. And this mystical receptor I was looking for would be like a lock on the cell, and it would fit in, and the adrenaline would then do something to that lock, open it, and things would happen in the cell. That was the idea. So how to prove this? Well, at first, there was no way to even study it.