So it moves back and forth relatively quickly. So it's closed here, open here, closed here, open here, and only when it's open can the actual substrate molecules, shown here in red, actually enter this central cavity of the transmembrane domain. So remember, we have the two transmembrane domains that essentially create this internal cavity that can fit that substrate molecule. Now, once the substrate molecule actually goes into this central cavity, that closes these two domains, and it also causes these two green domains the ATP binding cassettes to actually interact with one another. So it creates conformational changes in these two green structures and that increases their affinity for ATP molecules. And so in the next step, what happens is because we have these two cavities on the ATP binding consents that means two ATP molecules can actually fit into these two locations.