Generation of an Msx2-GFP conditional null allele.

Msx1 and Msx2, two members of the Msx gene family, encode homeoprotein transcription factors and play critical roles during mouse development. Because of the redundancy between the two genes, many of these roles can only be studied in double Msx1; Msx2 mutants. However, these animals die around 14.5 dpc, which precludes analysis of Msx gene function beyond this stage. Moreover, the pleiotropic defects displayed by these embryos make phenotypic analysis difficult. To overcome these restrictions and study the double Msx mutant phenotype at later stages, we generated an Msx2 conditional null allele using Cre/loxP technology. The strategy consisted of flanking the Msx2 gene coding sequence with two loxP sites. In addition, a green fluorescent protein (GFP) reporter gene was placed under Msx2 regulatory sequences in the modified locus. Our results demonstrate that the Msx2-GFP conditional allele behaves as a normal one, whereas Cre-mediated recombination creates an Msx2 null allele. With either allele, expression patterns of the GFP reporter gene and the Msx2 endogenous gene are identical.