About the Cover

About the Cover
Vertebrate skeleton patterning is elaborated by precisely controlled spatial expression of gene products during development. While the HOX transcription factors are key regulators of this process, downstream components are not well defined. In HOWELL et al. (pp. 699–707), a forward genetic screen in mice identified a mutation in a novel transmembrane protein-encoding gene that causes posterior-to-anterior homeotic transformations in the lower rib cage. In mutant skeletons (right), the normally freefloating eighth thoracic rib attaches to the sternum as would a normal seventh rib, and they show the beginnings of an extra rib on the first lumbar vertebra.