Besides the high similarity of the paired-domain sequence between murine and human Pax genes, mutations or deficiency of PAX1 and/or PAX9 have been associated with Jarcho–Levine and Klippel–Feil syndromes which are characterized by kyphoscoliosis or vertebral segmentation defects that phenocopy the Pax1−/−Pax9−/− mouse mutants (Bannykh et al., 2003; Lopez et al., 1997). This evidence concerns the gene PAX9 and Kyphoscoliosis.