Figure 1.

Gene targeting of Lmx1b. A, Strategy for generating floxed Lmx1b mice. Top, Wild-type locus. Exons 3–8 (black boxes) and introns (white boxes) are shown. The homeodomain is contained within exons 4, 5, and 6. Bottom, Targeting strategy. A loxP–PGKneobpA–loxP cassette (blue box with red triangles representing the loxP sites) was inserted into the first BglII site 5′ to exon 4, and an oligonucleotide containing a single loxP site was inserted into the first BglII site 3′ to exon 6 by conventional cloning methods. In both cases, the BglII site was destroyed. In the targeting construct, flanking regions were included (blue) that extended ∼3 kb 5′ to the PGKneobpA cassette and 5.5 kb 3′ to the 3′ loxP site and an MC1-TK cassette for negative selection. B, Southern blot shows a wild-type (WT) band (5.6 kb) and a floxed band (12.3 kb) after digestion with BglII.