Patterning of the vertebrate axial skeleton requires precise spatial and temporal control of Hox gene expression during embryonic development. MicroRNAs (miRNAs) are recently described modulators of gene activity, and members of the miR-196 and miR-10 families have been shown to target several Hox genes in vivo. Testing miRNA function in mice is complicated by potential redundancy between family members. To circumvent this, we have developed protocols for introducing modified antisense oligonucleotides (antagomiRs) in ovo during chick development. Using this approach, we identify a layer of regulatory control provided by the miR-196 family in defining the boundary of Hox gene expression along the anterior-posterior (A-P) embryonic axis. Following knockdown of miR-196, we observe a homeotic transformation of the last cervical vertebrae toward a thoracic identity. This phenotypic alteration is, in part, due to an anterior expansion of Hoxb8 gene expression and consolidates the in vivo relevance of post-transcriptional Hox gene regulation provided by miRNAs in the complex hierarchies governing axial pattering.