Sample time series of monomer and dimer copy numbers in genetic toggle switch. (a) MTF circuit, where monomer is the functional form of the repressor. (b) DTF circuit, where dimer is the functional form of the repressor. The left (right) column shows the number of the two monomer molecules A and B (dimers AA and BB), and the initial state is always with species A (red) in high abundance. Note that the switching frequency depends on the binding affinity of protein dimer.