So it's the transmembrane domain that is responsible for actually binding those calcium ions. And it's two calcium ions per one ATP that is actually pumped to the other side of that particular membrane. So in the e one state, we see that the ATP isn't actually bound and these calcium ions can move into the transmembrane domain, as shown here. And that's what happens when going from the E one stage to the E one calcium two stage. So now we have the two calcium ions inside the transmembrane domain. And now the ATP can actually go in and bind and interact with this green domain, the end domain.