And we have many other examples that we're going to look at. Now, the important part about step three is it creates an amplification of that signal. Why and what is that? Well, normally we have a low concentration of that primary messenger. We can have as low as a single molecule that binds onto that receptor, onto that transmembrane protein. And even though we only have this single primary messenger, what happens inside the cell is we produce many of these intracellular secondary messengers and then they can basically go on to carry out their individual processes.