To understand design principles underlying biological systems, simple oscillatory and switch-like systems have been constructed experimentally. Several systems based on E. coli [24,25] have been successfully demonstrated. Recently, components of the Lac and Ntr systems have been used to construct a genetic clock that upon change in connectivity, exhibits switch behavior [26]. After ignoring the dynamics of the read-out gene, the system of three equations for the concentrations of mRNA (x1, x3, x5) and proteins (x2, x4, x6) is given as,