Factor XIIIa–induced cross-linking of Aα251 and control fibrinogen. Factor XIII and thrombin were mixed, left on ice for less than 10 minutes, and added to fibrinogen at ambient temperature. The samples with final concentrations of 1 IU/mL factor XIII, 1 IU/mL thrombin, and 1.2 μM fibrinogen were incubated for the indicated times (minutes). The reactions were stopped by addition of SDS (1% final concentration) and incubation in boiling water for 5 minutes; samples were analyzed on 12% SDS-PAGE run under reducing conditions. The 0 time point samples contained only fibrinogen. Positions of molecular weight markers are indicated on the right; positions of bands corresponding to individual fibrin chains are indicated on the left. (A) Control fibrin. (B) α251 fibrin.