Generation of Ed knockout mice. A, Genomic organization of the 3′ region of the Igκ locus. The 1.2 kb HindIII-NdeI fragment shown as a shaded oval contains the entire Ed region. B, Targeting construct, with Ed and the neor gene flanked by three loxP sites. The HindIII-HindIII and NdeI-NdeI fragments were used as 5′ and 3′ homology arms. C, Targeted locus. Endogenous Ed was replaced with Ed-PGK-neor flanked by loxP sites. Probes are represented with thick lines. D, Ed deleted locus. Ed and the neor gene were deleted in vivo by Cre-loxP mediated recombination. E and F, Southern blot analysis of tail DNA. Tail genomic DNA was digested with NcoI and hybridized with probe A (E) or probe B (F). A 9.9 kb band derived from the WT allele and a 8.6 kb band derived from the Ed deleted allele were detected by Probe A or Probe B. A 5.0 kb band and a 6.2 kb band derived from the floxed allele were detected by probe A and probe B, respectively. NEB DNA markers were used for gel calibrations to determine fragment sizes.