FXIIIa catalyzed γ dimer formation. At 0 time, thrombin (1 U/mL) was added to a mixture of FXIII (1 U/mL) and plasma (A) or recombinant (B) fibrinogen (0.38 mg/mL). Reactions were stopped at the indicated times by the addition of SDS and 2-mercaptoethanol and analyzed by SDS-PAGE on 10% gels. 0 minute controls were prepared by adding thrombin, FXIII and fibrinogen to the SDS and 2-mercaptoethanol buffer; 60-minute controls were as above, but without added FXIII.