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. Now, in many cases so it's not shown in this diagram, but it's shown in this diagram. And we'll talk about that in just a moment. In many cases, the seven TM domain actually contains additional protein domains attached onto the intracellular side of that structure, as we see in this particular case.