A Vertebrate Story

In recent research in Portugal, findings show that making ribs is a process in which balances the activities of the known Hox genes. Their study shows that the two groups of Hox genes, Hox10 and Hox6, balance each other out with one promoting the growth of ribs while the other blocks it. They went further with the experiment finding that the production of ribs as well as muscles are all done through the same mechanism, in that muscles are needed for the ribs.

http://www.biologynews.net/archives/2010/04/27/to_have_or_not_to_have_ribs_a_vertebrate_story.html