Remember, a dimer is simply a molecule that consists of two polypeptide chains. Now, because alpha one and because alpha one and alpha two are identical and beta one and beta two are identical, we see that alpha one beta one dimer is identical to alpha two, beta two dimer. And we'll see in just a moment why we break it down into these two dimers instead of breaking it down into four individual polypeptide chains. Now, let's begin by recalling some information about hemoglobin that we learned previously. So we said the fact that the oxygen binding curve for hemoglobin is sigmoidal basically implies that hemoglobin behaves in a cooperative fashion. And what that means is the different heme groups actually interact with one another.