Table 1.

Comparison of theory and simulation for a constitutively expressed gene

η	Protein nos.		Protein concentration
	Simulation	Theory	Simulation	Theory
ηtot	0.26  ± 0.003	0.26	0.15  ± 0.005	0.15
ηint	0.15  ± 0.007	0.15	0.15  + 0.007	0.15
ηext	0.21  ± 0.004	0.21	0.05  ± 0.01	0.04

Time is the only extrinsic value, and there is consequently almost no extrinsic noise in the protein concentration (because this varies little during the cell cycle; see Fig. 2). The intrinsic noise in both cases, calculated by Eq. 21 (with an appropriate expression for cell volume when needed), is a very good approximation to ∫dtη̂, which is found to be 0.15 ± 0.008 from simulation and 0.15 from integrating Eq. 17. Parameter values are published as supporting information on the PNAS web site, except k₀ = 0.01 s⁻¹. Values stated are mean results from 100 simulations and errors are ±1 SD.