So to go back to our bear example, when we see that bear we panic and we begin running. What happens is or one of the things that begins happening is the endocrine system and more specifically the adrenal glands that on top of the kidneys begins producing a specific type of signal molecule we call epinephrine. Epinephrine diffuses into the bloodstream then goes around the cells and so the concentration of epinephrine around the cells increases and that ultimately influences the cells to carry out specific types of processes that lead to that physiological response of running away. 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.