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Illustration representing the gene regulatory network (GRN) of the bacterium Escherichia coli. The GRN is a network of nucleic acids, transcription factors and proteins that interact to control gene expression within a cell. It allows a bacterium to respond to its environment, for example by switching on genes to process a new food source. (KEYSTONE/SCIENCE PHOTO LIBRARY/EQUINOX GRAPHICS/SCIENCE PHOTO LIBRARY)