Systems that admit a treatment with the system size expansion may be described by a probability distribution P ( X , t ) , giving the probability of observing the system in state X at time t . X may be, for example, a vector with elements corresponding to the number of molecules of different chemical species in a system. In a system of size Ω Ω (intuitively interpreted as the volume), we will adopt the following nomenclature: X is a vector of macroscopic copy numbers, x = X / Ω Ω is a vector of concentrations, and ϕ ϕ is a vector of deterministic concentrations, as they would appear according to the rate equation in an infinite system. x and X are thus quantities subject to stochastic effects.