So Thrombin is a serum protease that uses proteolytic activation to activate fibrinogen. This molecule and the way that it activates it is by cleaving at four different sites on that molecule and by cleaving at these four different sites we basically remove four different peptides. And these four peptides are shown here. These are known as fibrino peptides and what we ultimately form is a fibrin monomer that consists of these three different subunits. So alpha, B and gamma. And we have twice the number of these.