Figure 1.

Regulatory functions used in the simulations. (a) The Hill function describes the dose-responce relationship between the amount of a regulator x₁ and the relative production rate of the regulated gene. The threshold parameter θ₁ = 2 is the same for all four curves. (b) Regulatory function for two regulators x₁ and x₂. Two Hill functions are combined by the algebraic equivalent of the Boolean AND function (see Table 1). The parameter values used in this panel are θ₁ = θ₂ = 2 and p = 10.