To clarify the function of the Hoxc-9 gene, we have introduced a mutation into the Hoxc-9 locus by gene targeting. Skeletal analysis of homozygotes revealed that anterior homeotic transformation occurred from the 10th thoracic vertebrae to the first lumbar vertebra. Bending and fusion of the ribs were observed. Eight or nine pairs of ribs were attached to the sternum. The sternum showed an abnormal pattern of ossification. Phenotypes of the mutant mice resembled those of the Hoxc-8 mutant mice. The data suggested that a functional interaction between the Hoxc-8 and Hoxc-9 proteins is involved in segmental determination. Moreover, changes in the Hoxc-8 gene expression pattern suggested that the Hoxc-9 gene regulates expression of the Hoxc-8 gene.