Cdxl is expressed along the embryonic axis from day 7.5 postcoitum until day 12, by which time the anterior limit of expression has regressed from the hindbrain level to the forelimb bud region. To assign a functional role for Cdx7 in murine embryonic development, we have inactivated the gene via homologous recombination. Viable fertile homozygous mutant mice were obtained that show anterior homeotic transformations of vertebrae. These abnormalities were concomitant with posterior shifts of Hox gene expression domains in thesomitic mesoderm. The presence of putative Cdxlbinding sites in Hox gene control regionls as well as in vitro transactivation of Hoxa-irindicates a direct regulation.