And any time a primary messenger wants to bind onto the structure, it binds on the outside portion the extracellular side of this seven TM receptor. And so right here, we have an internal cavity that can fit that primary messenger. So the primary messenger could be a neurotransmitter. It could be a hormone, as we'll see in this particular case. It can even be a synthetic drug that we create in the laboratory to basically inhibit the activity of the signal transduction pathway. In either case, it always binds onto the extracellular side of that receptor, and it doesn't actually move into the cytoplasm of that cell.