So, let's think back to hemoglobin. So, when we discussed hemoglobin, we discussed what it means for an enzyme or a protein to behave in a cooperative fashion. So, in our discussion on hemoglobin, we said that hemoglobin behaves cooperatively because it consists of different subunits. And so it has different active sites. It has more than one active side. It has more than one side to which the oxygen actually binds to.