Targeted disruption of the murine F. IX gene by homologous recombination. An 18.6-kb genomic map of the mouse F. IX gene showing exons f, g, and h. The targeting vector was made from the 11.6-kb Xho I-Not I fragment of the lambda clone. The targeted allele contained a neo gene inserted into exon g and exon h. The 3.2-kb BamHI fragment covering exons g and h and the introns was replaced by a 1.6-kb neo^r cassette. The 2.1-kb thymidine kinase cassette was cloned into the Xho I site. A 0.5-kb fragment (Probe A), external to the Xho I site of the targeting vector, and an HindIII-Not I 1.5-kb fragment (Probe B) were used to screen the ES cell clones and mouse tail DNA. A 637-bp neo gene probe was also used to screen the same clones (data not shown). B, BamHI; E, EcoRI; H,HindIII; N, Not I; X, Xho I; Xb, Xba I; TK, thymidine kinase gene; neo, neomycin gene. The dotted line represents a portion of the genomic map, which was not included in the λ clone, but was identified by subsequent mapping after Southern blot hybridization of ES cell and mouse DNA. The * (asterisk) on theNot I site denotes that it is not part of the clone λ.