Strategy for PCR amplification of human FXI mRNA. The FXI cDNA is encoded by 15 exons. Three sets of oligonucleotide primers (Table 1) were designed to amplify a 400 bp fragment from exons 3 to 6 of the heavy chain (primers 1 and 2), a 552 bp fragment from exons 10 to 14 of the C-terminus of the heavy chain and the catalytic light chain (primers 3 and 4), and a 1,673 bp fragment from exons 3 to 15 covering the coding region for the mature plasma protein (primers 5 and 6). A1 to A4 designate the four apple domains of the heavy chain; E, exon.