So even though in this case it's a single polypeptide chain, it contains four different domains. Two of these domains are shown here, they're essentially the transmembrane domains and the other two domains are shown here. They're called the ATP binding domains or ATP binding cassettes. So the MDR protein consists of four domains. Two are membrane spanning domains shown in brown and the other two are ATP binding domains, also known as ATP binding cassettes or simply ABCs because this is where the ATP will actually bind, as we'll see in just a moment. Now, it turns out we have many different types of examples of ABC transporters in our own cells as well as other eukaryotic cells and even prokaryotic cells.