Biology
The genetic toggle (Gardner et al. 2000) consists of two mutually repressing genes. Protein A represses gene B; protein B represses gene A. The system is bistable: one protein dominates. Molecular noise (Gillespie dynamics) drives spontaneous switching — rare events but real.
Model
Gillespie SSA with production rate α/(1+(B/K)^n), degradation rate δ·A for each protein. Deterministic fixed points at A-high or B-high. Noise smears the distribution and enables switching.