Western blots of recombinant murine factor XI activated by (A) thrombin and (B) factor XIIa in the presence of dextran sulfate. Murine factor XI (5 μg/mL) was incubated with 1 μg/mL dextran sulfate and either human thrombin or factor XIIa (2.5 nmol/L) at 37°C. At various time points, samples were removed and mixed with SDS-reducing sample buffer. Western blots were prepared as described in Materials and Methods. The positions of molecular weight markers in kD are shown at the left of the figure. Abbreviations: fXI, factor XI zymogen; fXIc, factor XI cleaved; HC, factor XIa heavy chain; and LC, factor XIa light chain.