Fig. (4). Loss of Hox10 rib-suppressing activity in the trunk of the snake.

In the mouse rib-bearing thoracic and rib-less lumbar domains are determined by expression of Hox6 and Hox10 genes (for the apparently redundant role of Hox6 genes in rib induction see main text). Expression of Hox6 genes in the PSM leads to activation of Myf5 and Myf6 in the hypaxial domain of the somites resulting in rib formation. Expression of Hox10 genes at the time the lumbar somites are formed disrupts this cascade and represses Myf5 and Myf6 resulting in rib-less vertebrae [9]. In snakes Hoxa10 and Hoxc10 are strongly expressed in the posterior thorax, nevertheless this region develops ribs wich are identical to ribs located more anteriorly, outside of the Hox10 expression domain [4,5].