The network of a toggle switch. (a) The topology of the network and variables representing the reaction rates. Single copies of gene A and gene B in the chromosome each encode a protein product. Two protein monomers can repress the transcription of the other gene. The synthesis of protein product of gene A and B depends on the bound or unbound state of the gene. (b) The chemical reactions of the 8 stochastic processes involved in the toggle-switch network. The reaction rates include s for protein synthesis, d for protein degradation, b for protein-gene binding, and u for protein-gene unbinding.