Timed collinear activation of Hox genes during gastrulation controls the avian forelimb position

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the sequence limb-forming/non-limb-forming domains is embedded in the collinear organization of Hox genes through their specific activation/repression function on limb initiation, it is in turn the timing of collinear activation that sets the relative position of these domains along the main axis.

 

once Hox genes have set the forelimb field of variable size in different species, prepatterning of the early mesenchyme by Hox5, Hox9 and most likely other Hox genes could in turn induce variation in the position of the ZPA which could be responsible for variation in the definitive position of the forelimb\