The morphologies of different regions of the vertebral column are regulated by Hox genes. (a) In the mouse, six lumbar vertebrae form just anterior to the sacral vertebrae (numbers in red). (b) In mice lacking the function of the posteriorly acting Hoxd11 gene and possessing one functional copy of the Hoxa11 gene, seven lumbar vertebrae form and one sacral vertebra is lost. (c) In mice lacking both Hoxa11 and Hoxd11 function, eight lumbar vertebrae form and two sacral vertebrae are lost.
[Photographs courtesy of Dr. Anne Boulet, HHMI, University of Utah; from S. B. Carroll, J. K. Grenier, and S. D. Weatherbee, From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design, 2nd ed. Blackwell, 2005.]