We analyze a class of chemical reaction networks under mass-action kinetics and involving multiple time-scales, whose deterministic and stochastic models display qualitative differences. The networks are inspired by gene-regulatory networks, and consist of a slow-subnetwork, describing conversions among the different gene states, and fast-subnetworks, describing biochemical interactions involving the gene products. We show that the long-term dynamics of such networks can consist of a unique attractor at the deterministic level (unistability), while the long-term probability distribution at the stochastic level may display multiple maxima (multimodality). The dynamical differences stem from a novel phenomenon we call noise-induced mixing, whereby the probability distribution of the gene products is a linear combination of the probability distributions of the fast-subnetworks which are 'mixed' by the slow-subnetworks. The results are applied in the context of systems biology, where noise-induced mixing is shown to play a biochemically important role, producing phenomena such as stochastic multimodality and oscillations.In this section, chemical reaction networks are defined [14,17,18,19], which are used to model the biochemical processes considered in this paper, together with their deterministic and stochastic dynamical models. We begin with some notation.Definition 2.1 Set R is the space of real numbers, R ≥ the space of nonnegative real numbers, and R > the space of positive real numbers. Similarly, Z is the space of integer numbers, Z ≥ the space of nonnegative integer numbers, and Z > the space of positive integer numbers. Euclidean vectors are denoted in boldface, x = (x 1 , x 2 , . . . , x m ) ∈ R m . The support of x is defined by supp(x) = {i ∈ {1, 2, . . . , m}|x i = 0}. Given a finite set S, we denote its cardinality by |S|.Definition 2.2 A chemical reaction network is a triple {S, C, R}, where (i) S = {S 1 , S 2 , . . . , S m } is the set of species of the network.