Gene networks are some of the most basic features of a living organism. An external or internal stimulus activates some genes, which in turn control others genes whose activity turns on or off various biological processes (such as the cell cycle, energy production, DNA repair, cellular suicide etc).
Many of the regulatory functions are controlled by attachment of special proteins (transcription factors) to 6 - 10 nucleotide long binding sequences located on the DNA, activating or suppressing expression of the regulated gene. Our ability to identify these binding sites is essential to understand the way biological networks operate.
As the genomes of various organisms became known, it turned out that complex and simple organisms differ less than anticipated in the sizes and makeup of their genomes; complexity of an organism is now believed to be reflected mainly in the manner in which expression is regulated. According to consensus, transcription of human genes is regulated predominantly by factors that bind to sites whose distances from the transcription start site may vary widely and reach tens of thousands of base pairs.