So we see that a protein kinases are used by all four pathways that we discuss, and this implies that they play a crucial role in this signal transduction pathway and actually carrying out that particular type of physiological effect. Now, secondary messengers are also used by each one of these four different types of pathways. And we said that all these signal pathways use these secondary messengers to basically amplify that initial signal. So these secondary messengers are intracellular agents, they could be molecules or ions whose concentration can be greatly amplified, thereby amplifying the overall signal that was initially taken up by that cell. So these secondary messengers typically act on proteins or enzymes that play crucial roles in the signal transduction pathways. And so in our discussion, these are the secondary messengers that we discussed, except in this particular case, we actually didn't discuss how calcium or cyclic Amp is used by this pathway.