  Spatial patterns in the early fruit fly embryo emerge from a network of
interactions among transcription factors, the gap genes, driven by maternal
inputs. Such networks can exhibit many qualitatively different behaviors,
separated by critical surfaces. At criticality, we should observe strong
correlations in the fluctuations of different genes around their mean
expression levels, a slowing of the dynamics along some but not all directions
in the space of possible expression levels, correlations of expression
fluctuations over long distances in the embryo, and departures from a Gaussian
distribution of these fluctuations. Analysis of recent experiments on the gap
genes shows that all these signatures are observed, and that the different
signatures are related in ways predicted by theory. While there might be other
explanations for these individual phenomena, the confluence of evidence
suggests that this genetic network is tuned to criticality.
