We have described a collinear pattern of Hox expression along the primary axis in snake and caecilian embryos.
At the anterior and posterior regionalized parts of the axis we find that Hox gene expression coincides with expected anatomical transitions.
Genes, such as HoxA3, HoxB4, HoxA6 (in snake only), and HoxC13 have anterior expression boundaries corresponding to clear morphological transitions in the axial skeleton (atlas, axis, rib-bearing region and dorsal–caudal transition).