The potential of the vertebrate limb as a model system to study developmental mechanisms is particularly well illustrated by the analysis of the Hox gene network. These genes are involved in the establishment of patterns and encode positional information. Their functional organisation during both limb and trunk development are very similar and seem to involve the progressive activation in time, along the chromosome, of a battery of genes whose products could differentially instruct those cells where they are expressed. Thus, cells expressing different Hox genes may behave differently and therefore give rise to various related, but different, structures. The asymmetric distribution of the Hox transcript during limb development likely reflects the final limb asymmetry