In this issue of Cell, Kalir and Alon (2004) describe the construction of an in silico model for the regulatory network responsible for the control of flagellar biosynthesis in E. coli based on quantitative gene expression data. They show how the model can be used as a quantitative blueprint to design genetic modifications with predictable influence on the dynamics of the system. The work by Kalir and Alon (2004) provides a general approach for building detailed models of transcriptional regulatory networks.