Immunoprecipitation of FXI Δ6/7 isoform from normal plasma. FXI present in plasma and in media of COS-1 cells transfected with pcDNA3/FXI, or pcDNA3/FXI-Δ6/7 was immunoprecipitated using both goat anti–human FXI antibody and mouse anti–human FXI (data not shown). Samples were analyzed on 6% nonreducing SDS-PAGE, and Western blotting was performed using polyclonal goat anti–human FXI peroxidase-conjugated IgG. The pcDNA3 empty vector was used as negative control (mock). + represents presence of goat anti–human FXI antibody; −, absence of goat anti–human FXI antibody; CTR+, 2 ng of commercially available human FXI; and input, 10 μL of normal plasma before the immunoprecipitation step.