Exon 28 was replaced with one in which a single base pair modification (CTG to ATG) resulted in a valine to methionine substitution at amino acid 1316 (V1316M). A neomycin selection cassette flanked by loxP sites was inserted upstream of exon 28. This mutation is found in von Willebrand's disease type 2B patients. Cre-mediated recombination removed the neomycin cassette.