The Emx1 gene is a mouse homologue of a Drosophila head gap gene, empty spi
racles. Corpus callosum defects have been reported for mutant mice lacking
the Emx1 gene, indicating a possible involvement of the Emx1 gene in the re
gulation of axon guidance during development. However, it has recently come
to light that genetic background could influence the outcome of the corpus
callosum defects in mutant mice generated by ES cell technology. To invest
igate whether the corpus callosum defects resulted directly from Emx1 gene
inactivation, we backcrossed the Emx1 mutant mice into the C57BL/6 backgrou
nd and found that the Emx1 homozygous mutant mice with the C57BL/6 backgrou
nd showed a normal corpus callosum score even though the cross-sectional ar
ea was 8% less than that of their wild-type littermates, indicating that ge
netic background could change the pattern of corpus callosum defects in Emx
1 mutant mice. Furthermore, the indusium griseum and taenia tecta were alwa
ys present and the cerebral cortical layers were well differentiated in Emx
1 mutant mice. These results suggest that inactivation of the Emx1 gene doe
s not contribute directly to the defects of corpus callosum and other brain
structures associated with Emx1 mutant mice derived from a 129/Sv backgrou
nd. (C) 2000 Academic Press.