FXIIIa-catalyzed cross-linking of fibrinogen. Cross-linking of fibrinogen by FXIIIa was examined with 8% SDS-PAGE, under reducing conditions as indicated for Figure 4, except that hirudin was added to FXIIIa prior to addition to fibrinogen and no thrombin was added to fibrinogen. The reduced fibrinogen chains (Aα, Bβ, γ, cross-linked γ-γ dimer, and cross-linked α-chain polymers) are indicated on the right side of the gels.