Figure 3. Msx2Cre removal of Sp8 on a Sp6 deficient background.

The external aspect (top row) and skeletal preparations of the forelimb (middle row) and hindlimb (bottom row) of newborns are shown for each genotype (genotypes indicated at the top). Msx2Cre conditional removal allows transient expression of Sp8 in both forelimbs and hindlimbs which results in Sp8 conditional mutant (D–F) displaying a milder limb phenotype than ubiquitous mutants (A–C). One single conditional allele of Sp8 in the forelimb (G–H) seems to be equivalent to both functional alleles of Sp6 (A–B) while in the hindlimb is not sufficient for limb development (C, I). This conditional allele of Sp8 in addition to one single allele of Sp6 permits the formation of the three PD segments of the limb although with a single digit (J–L). Finally, one conditional allele of Sp8 plus a normal allele Sp8 results in SHFM (M–O).