Metabolic labeling of wild-type and Δ6/7 mutant recombinant FXI in transiently transfected COS-1 cells. Twenty-four hours after transfection, cells were labeled with [³⁵S]-cysteine and methionine mixture as described in “Expression and quantitation of recombinant Δ6/7 FXI.” Radiolabeled FXIs were immunoprecipitated from cell lysates and media using goat anti–human FXI. Immunoprecipitated FXI was analyzed on 8% reducing (bottom panel) and 6% nonreducing (top panel) SDS-PAGE and visualized by scanning phosphor image screens. The pcDNA3 empty vector was used as negative control (mock). + indicates presence of goat anti–human FXI antibody; −, absence of goat anti–human FXI antibody; and arrowheads, homodimeric molecules in nonreducing panels. In the same panels, the smears corresponding to the heterodimers and the Δ6/7 homodimers are indicated by square brackets.