A large number of CNS was identified within the Hox clusters and outside the Hox clusters spread over large regions. The CNS include previously characterized enhancers and overlap with the 5' global control regions of HoxA and HoxD clusters. Most of the CNS are likely to be control regions involved in the regulation of Hox and other genes in these loci. We propose that the regulatory elements spread across large regions on either side of Hox clusters are a major evolutionary constraint that has maintained the exceptionally long syntenic blocks at the HoxA and HoxD loci