Simplified description of the interaction between activated FXIII and fibrin(ogen). Thrombin induces release of AP-FXIII and exposure of the AP-FXIII cleft. The fibrin αC region binds to the AP-FXIII cleft leading to an ideal position for cross-linking of fibrin chains by the FXIII active site. The same mechanism may also apply to other FXIII substrates.