Fibrinogen molecules with location of the γ′ chain. The amino acid extension of the γ′ chain is located at the carboxy-terminus (black arrow) of the γ chain on the D region of the molecule. When fibrinogen is converted to fibrin by thrombin cleavage of fibrinopeptides in the E region, fibrin polymerizes through D-E-D interactions, with the γ′ extension at the D-D interface. The structure of the γ′ chain is unknown, but an extra 16 amino acid residues may extend up to 30 to 40 Å or more depending on folding. Considering this length, the γ′ extension could stretch to the E region of a neighboring molecule in the D-E-D complex.