FXIIIa-catalyzed cross-linking of fibrin. Cross-linking of fibrin by FXIIIa was examined with 8% SDS-PAGE, under reducing conditions as described in “Materials and methods.” Fibrinogen (0.47 mg/mL) was mixed with FXIIIa (3.3 U/mL) and thrombin (0.07 U/mL), and the reaction was incubated for the specified time at 37° C in 20 mM HEPES, pH 7.4; 0.12 M NaCl, and 0.67 mM CaCl₂ buffer. The reduced fibrin chains (α, β, γ, cross-linked γ-γ dimer, and cross-linked α-chain polymers) are indicated on the right side of the gels.