Fig. 4. Defects in the rib cage of Hox mutant mice show functional redundancy. Ventral views of the skeletal phenotypes from wild-type (A), Hox5 five-allele (B), Hox6 five-allele (C), Hox9 seven-allele (D) and Hox5/Hox6 trans-triple heterozygous (E) mice. Numbers below the rib cage indicate the thoracic vertebrae with ribs that fuse to the sternum. The red bar is included as a size reference. Note that any combination of five mutant alleles of Hox5 or Hox6 (Aabbcc, aaBbcc and aabbCc) shows very similar phenotypes to the ones shown in B and C. In the Hox9 paralogous group, however, Hoxc9 contributes more strongly to extra sternal rib growth and fusion than the other Hox9 genes (data not shown).