Targeted TXAS gene deletion strategy. (A) Exons 8 to 10 are depicted as open boxes. Filled arrows are the HPRT and tk genes. Transcription of HPRT gene is opposite to that of TXAS gene. Also shown are the hybridization probe A (5′), a 2-kb XbaI-XbaI fragment, and probe B (3′), a 0.6-kb XbaI-PstI fragment. The length of the DNA fragments of WT TXAS hybridized by the probes is indicated at the top and that of recombinant deletion mutant is indicated at the bottom. (B) Southern blot analysis of ES cell DNA digested by EcoRV and hybridized by probe A (left) or digested by BstXI and hybridized by probe B (right). The size of the bands for WT and mutant (KO) alleles is shown. Lane 1 shows an ES cell clone with WT and mutated bands, whereas lanes 2 to 4 show WT clones.