Fig. 1. Conventional and to-scale representations of Hox clusters. The colored boxes correspond to the various paralogy groups - those genes that are most closely related in sequence, and hence are derived from a common ancestral Hox gene. (A) Schematic depicting Hox clusters as they are usually represented in the literature and textbooks. It shows the respective positions of paralogy groups 1 to 14 for the cephalochordate amphioxus and a vertebrate prototypic cluster, and their corresponding Drosophila genes located on both ANT-C and BX-C. (B) A more precise representation of the organization within Hox clusters, with the correct relative distances and a clear separation present between the two Drosophila sub-clusters. The comparison between A and B highlights the differences that exist between a structural reality and its conceptual interpretation. Only the former, as shown in B, should be considered when discussing the structural and functional evolution of Hox clusters. ANT-C, Antennapedia complex; BX-C, Bithorax complex.