General phylogenetic tree of deuterostomes showing the Hox clusters of non-olfactores deuterostomes known to date. The Hox repertoire of a substantial number of groups within the invertebrate deuterostomes is still lacking (black question marks), and the origin of a 15 Hox gene cluster in cephalochordates, or when the Hox4 was lost in echinoderms are still a mystery (indicated by red question marks and arrows). Yellow, anterior Hox genes; orange, Hox3; blue, central Hox genes; green, posterior Hox genes.
Pascual-Anaya et al. BMC Developmental Biology 2013 13:26 doi:10.1186/1471-213X-13-26