The targeting vector replaced exons 1-3 with exon 1, an FRT-flanked neomycin selection cassette, a loxP site, exon 2, exon 3 containing three amino acid changes [substitution of proline at amino acid 64 to leucine (p.P64L), lysine at amino acid 72 to glutamine (p.K72E), and isoleucine at amino acid 73 to histidine (p.I73H)], and a second loxP site. The three amino acid substitutions in exon 2 were introduced to obtain the murine ortholog of human PF4. (J:270132)