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Fig. 1. Disruption of Tbx6 binding sites eliminates Mesp2
expression. (A) Targeting strategy to generate the Mesp2
enhancer knockout mouse (P2EmB1D). A DNA fragment containing mutated
Tbx6 binding sites (black ovals with X) was substituted for the wild-type
sequence (white ovals) by homologous recombination. The PGK-neoR selection
marker was removed by the Cre-loxP system to obtain a
neo allele.
(B) PCR detection of homozygotes in the P2EmB1D intercross.
(C) Impaired skeletal segmentation in the Mesp2 enhancer
knockout mouse. The P2EmB1D/P2EmB1D mouse exhibits severe skeletal
malformation at E17.5 (centre) identical to that of the Mesp2-null
mouse (P2MCM/P2MCM, right). Note the shortened spine with
incompletely segmented vertebrae (upper panels) and fused ribs (bracket in
lower panels). (D) Expression of Mesp2 and the somite-specific
genes Mesp1, Epha4 and Tbx18 in P2EmB1D/+ (left
column) and P2EmB1D/P2EmB1D (right column) embryos. Mesp2
mRNA expression is eliminated in the P2EmB1D/P2EmB1D homozygotes.
Wild-type (+/+) and heterozygote (P2EmB1D/+) embryos showed varying
Mesp2 expression patterns owing to its cyclic expression.
Mesp1 is upregulated and Epha4 is not affected, whereas
Tbx18 is completely abolished in P2EmB1D/P2EmB1D.