Well, basically it's a protein that contains six subunits. Five of these subunits are identical and the other one is different. So five of these units are known as the B chains and the other unit is known as the A chain. The A chain is shown in green and this is the enzymatic. It's the catalytic A chain that will cause its effect once it gets into the cell, as we'll see in just a moment. And these five binding b chains basically create this cyclical structure that ultimately is used to actually bind into a special region of the membrane of the intestinal epithelial cells.