 These basic “cis-regulatory” units are organized into systems of interacting modules. Because transcription factor proteins are products of gene expression, the regulatory modules that control their expression are sites of “primary core control” for development. These core control modules form an interconnected network, reflecting the influences of transcription factor proteins on one another’s expression. All organisms have a genetic regulatory network composed of DNA sequences distributed throughout its genome, which constitutes a stable underlying program for development. Collectively, DNA modules act as “a vast, delocalized computational device” that processes regulatory states of a cell (Davidson 2006: 185). Furthermore, regulatory DNA sequences are (for the most part) invariant across cells of an organism, and organisms of a species, with a relatively small set of “hub” transcription factors conserved throughout much of the animal kingdom. The regulatory genome therefore offers a unified explanation of animal evolution and development. However, there are good reasons to think the role of DNA sequences in gene expression is not distinctive, causally or informationally (see the entries on  gene,   genetics, and  reductionism in biology).   Other recent theoretical approaches tend toward more inclusive, less gene-centric explanations of development.