SDS-PAGE analysis of factor XIa activation of plasma factor IX and wild-type factor IX. Factor IXpl (A) or IXwt (B) (2.7 μmol/L) was incubated with factor XIa (5 nmol/L) in TBS containing 10 mmol/L Ca²⁺ at 37°C. At discrete time intervals, aliquots (3 μg) of the reactions were resolved on 12.5% SDS-PAGE under reducing conditions and stained with Coomassie Blue.