  We construct a gene network based on expression data from DNA microarray
experiments, by establishing a link between two genes whenever the Pearson's
correlation coefficient between their expression profiles is higher than a
certain cutoff. The resulting connectivity distribution is compatible with a
power-law decay with exponent ~1, corrected by an exponential cutoff at large
connectivity. The biological relevance of such network is demonstrated by
showing that there is a strong statistical correlation between high
connectivity number and lethality: in close analogy to what happens for protein
interaction networks, essential genes are strongly overerpresented among the
hubs of the network, that is the genes connected to many other genes.
