How single-gene noise creates cell-to-cell variability in protein levels
Genes switch stochastically between an active (ON) and inactive (OFF) state. mRNA is produced only in bursts during ON periods, then degraded.
The protein copy number follows a negative binomial distribution in steady state. The key parameter is the burst frequency b = k_on/k_off and burst size B = k_m/k_off.
This is the source of non-genetic variability (noise) in identical cells.