Neighbor-joining phylogeny of PRD class genes based on homeodomain sequences. Pairwise distances between homeodomains were calculated using the PAM matrix in the computer program protdist in the Phylip package (Phylogenetic Inference Package, V.3.61). Abbreviations for taxa are as in Figure 1. Numbers above nodes indicate the percentage of 1000 bootstrap replicates in which the given node was recovered in a neighbor-joining analysis. The tree is rooted using Hox1 and distalless homeodomains from the lancelet and the fruit fly.