Tandem homeobox gene duplication and recruitment to body patterning. Tandem duplication of homeobox genes in early metazoan evolution generated ProtoNK and ProtoHox genes (black boxes), plus neighbouring homeobox genes (grey boxes). The ProtoHox gene cluster then duplicated to generate Hox and ParaHox gene clusters. It is proposed that the NK gene cluster, Hox gene cluster and ParaHox gene cluster were recruited to pattern the mesoderm, ectoderm (including central nervous system) and through-gut of the first bilaterian animals. Adapted from .