A single loxP site was inserted into intron 4 and a loxP-flanked neomycin-thymidine kinase selection cassette into intron 5 of the gene. The loxP sites flank a region of the gene encoding most of the first kringle domain necessary for the biological activity of the corresponding protein. Transient Cre recombinase transfection removed the selection cassette prior to the production of chimeric mice.