So two three BPG, or simply two three biphosphoglycerate, is this biological molecule that acts as an allosteric effector of heat hemoglobin. So what it does is it binds onto the hemoglobin molecule at a special location that is not the same location that oxygen binds to, and it stabilizes the T state of that hemoglobin. And by stabilizing the T state, it lowers the affinity of hemoglobin for oxygen. So it basically shifts the entire curve, the oxygen binding curve to the right side with respect to pure hemoglobin that does not contain the two three BPG. Now, how do we say that two three BPG binds onto hemoglobin? So what we said was at the center of the hemoglobin is this positively charged pocket, is this region of space that contains a positive charge or positive charges.