Now, epinephrine is not the only example of this signal molecule that initiates the signal transduction pathway. We have many, many other examples. And two other examples that we're going to study in detail in the lectures to come are insulin and the epidermal growth factor. But we have many other examples. For instance, other hormones are also these signal molecules. Now, what I'd like to do in this lecture is generalize the steps of the signal transduction pathway.