Snake

Snakes are discussed as a dramatic example of how changes in Hox gene expression patterns - not changes to the genes themselves - can produce major body plan modifications. Snake body plans evolved through changes in Hox gene expression that extended trunk identity along most of the body axis (producing hundreds of vertebrae) while suppressing limb development in regions where other tetrapods develop forelimbs. Crucially, snakes retain functional Hox genes; they've simply changed the regulatory logic determining where these modular developmental programs activate.