Fibrinogen is a 340 kDa glycoprotein found in the blood plasma of all vertebrates. It is transformed into a fibrin clot by the action of thrombin. Recent X-ray structures of core fragments of both fibrinogen and fibrin have revealed many details about this polymerization event. These include structures of a 30 kDa recombinant gamma C domain, an 86 kDa fragment D from human fibrinogen and a cross-linked double-D fragment from fibrin.