Figure 1

a, E. coli expressing two identical promoters driving two different fluorescent proteins, in red and green, respectively. Because of noise, the ratio of red to green intensity differs from cell to cell²². b, A clonal population of B. subtilis cells differentiate into different fates in the same conditions. Here, some cells grow vegetatively or sporulate (green fluorescence), others have completed sporulation (white), and one has differentiated into a state of genetic competence (red fluorescence). Image provided by G. Süel. c, Mouse embryonic stem cells show relatively homogeneous expression of Oct4 (red nuclear protein staining), but heterogeneous expression of Nanog (green nuclear protein staining). Image provided by F. Tan. d, The C. elegans skn-1 mutant shows noise-driven partial penetrance. Two genetically identical embryos are shown. One has developed a gut (elt-2 RNA staining, red) whereas the other has not (nuclei in blue). Image provided by A. Raj. e, Mechanisms that shape noise in gene expression. Noise is characterized by bursty expression of mRNA (top). Proteins typically have longer lifetimes than bursts, leading them to time-average or `buffer' these bursts (middle). Finally, noise in one gene can propagate to generate further noise in the expression of downstream genes (bottom).