Small-number effects: a third stable state in a genetic bistable toggle switch.

A genetic toggle switch was studied experimentally and theoretically. We have found an additional kinetic stable state where all the genes express very lowly, which is predicted to be unstable by dynamical systems theory. It can also stably coexist with the other two known stable states, although this coexistence is forbidden in a deterministic dynamical system. We analyze that this nontrivial phenomenon results from the discrete and fluctuate nature in such a small system, by comparing experimental results with modeling results of exact stochastic simulations and differential equations.