  Regulatory interactions between genes show a large amount of cross-species
variability, even when the underlying functions are conserved: There are many
ways to achieve the same function. Here we investigate the ability of
regulatory networks to reproduce given expression levels within a simple model
of gene regulation. We find an exponentially large space of regulatory networks
compatible with a given set of expression levels, giving rise to an extensive
entropy of networks. Typical realisations of regulatory networks are found to
share a bias towards symmetric interactions, in line with empirical evidence.
