Recombinant murine and human (A) factor XI and (B) factor XIa. Recombinant protein expressed in 293 fibroblasts was purified as described in Materials and Methods. Two microgram samples of each protein were size fractionated under reducing and nonreducing conditions on a 10% polyacrylamide SDS-gel, followed by staining with Coomassie brilliant blue. The positions of molecular weight markers in kD are shown at the left of the figure.