Two of these subunits are alpha subunits, and the other two subunits are beta subunits. So we have alpha one and alpha two that combines with beta one and beta two to form a protein tetrimer that we call the dull hemoglobin. So this is what it looks like. We have alpha one, alpha two shown in green, and we have beta one and beta two shown in orange. Now, when we form the adult hemoglobin, the adult hemoglobin actually contains a cavity inside a space that is capable of binding a molecule called two three BPG. Now, what exactly is two three BPG?