It is generally assumed that the characteristic deregionalized body plan of species with a snake-like
morphology evolved through a corresponding homogenization of Hox gene expression domains along the
primary axis. Here, we examine the expression of Hox genes in snake embryos and show that a collinear
pattern of Hox expression is retained within the paraxial mesoderm of the trunk. Genes expressed at the
anterior and most posterior, regionalized, parts of the skeleton correspond to the expected anatomical
boundaries. Unexpectedly however, also the dorsal (thoracic), homogenous rib-bearing region of trunk, is
regionalized by unconventional gradual anterior limits of Hox expression that are not obviously reflected in
the skeletal anatomy