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. So both eukaryotic cells and prokaryotic cells, such as bacterial cells actually have ABC transporters but usually there are two important differences. So number one, eukaryotic cells usually contain ABC transporters that consist of a single polypeptide chain, a single polypeptide subunit, as we saw in the case of the MDR protein. So MDR proteins, found on the membrane of human cancer cells basically is a single polypeptide chain.