Although the cells of our body can use all sorts of different types of signal transduction pathways to initiate cellular processes that ultimately create some type of physiological effect, all these signal transduction pathways have four important properties in common. They share four important principles. And to demonstrate what these principles are, I'd like to focus in on the four different types of pathways pathways that we discussed so far. So in our discussions, we discussed epinephrine signaling, insulin signaling, EGF signaling, and we also looked at the phosphonosatide cascade. So what exactly are the four factors that these four pathways actually have in common? Well, fact number one is they all use protein kinases.