Engineering and analysis of inducible FX–deficient mice. (A) Mice containing an engineered f10 gene with loxP sequences surrounding exons 4 and 5. Upon activation of Cre-recombinase, both exons are excised. P1 and P2 indicate the locations of primers used for postinduction analysis. (B) Genomic DNA was isolated from liver of pI:pC–treated Cre⁺ and Cre^–f10^flox/flox mice. A virtually complete recombined F10 locus was observed in Cre⁺ mice. (C) A representative curve of RVV-X–induced FXa amidolytic activity over time in plasma from f10^WT mice (n = 3) and f10^low mice (n = 3). (D) Residual FX activity measured in RVV-X–induced activity assay. LOD of this assay is 0.5% FX. All mice having <1% FX activity are designated as f10^low mice. (E) Survival curve of of pI:pC–treated f10^WT mice and f10^low mice (both n = 7). No differences in survival after 12 months were observed. LOD, limit of detection.