(A) The DNA in cells is wrapped around proteins called histones (spheres) and which are then tightly packaged inside a structure called chromatin (left). The genes in the DNA cannot be expressed as proteins when the DNA is tightly packaged inside chromatin. In addition to genes (shown here in blue), the DNA contains cis-regulatory elements such as promoters (yellow) and enhancers (red), and the expression of a given gene is usually coordinated by a large number of these elements (although only two are shown for each gene here). In this example the fact that the distal element regulating gene B is adjacent to gene A is predicted to constrain the evolution of both genes. (B) The phylogenetic relationship of bilateria, eumetazoa, metazoa and choanoflagellates is shown (left), along with the genomic regulatory features found in each example species (right). All animals (metazoans) progress through development to multicellular adults, which distinguishes them from closely related unicellular organisms (such as the choanoflagellates). Bilateria, such as Drosophila melanogaster, have several regulatory features that are absent in related unicellular species. However, a number of these regulatory features have been characterized in basal eumetazoans (e.g. sea anemones) and now basal metazoans (e.g. Amphimedon queenslandica).