Western blot analysis of factor XIa activation of plasma factor IX and factor IXR94S using a factor IXa heavy chain antibody. Factor IXpl (A) or IXR94S (B) (2.7 μmol/L) was activated by factor XIa (18 nmol/L) in TBS containing 10 mmol/L Ca²⁺ at 37°C. The activation products were sampled at the indicated time intervals, resolved on reducing 12.5% SDS-PAGE, and transferred to Immobilon-P polyvinylidene fluoride (PVDF) membrane. The membrane was blotted with the C10D monoclonal antibody, which recognizes the heavy chain (HC) of factor IXa, and developed using chemiluminescence. The positions of intact factor IX, IX (HC + AP) and IXaHC are indicated. The positions of the molecular weight markers are indicated at left.