Figure 1: The asymmetric simple exclusion process is a toy model for describing nonequilibrium behavior. (Top) The model in the abstract. In an infinitesimal time window, a particle located on any one site hops to a site to its right with rate $p$, and to its left with rate $q$, provided these sites are empty. The lattice can exchange particles with the outside world, with particles entering or leaving the left end of the chain with rates $\alpha$ and $\gamma$, respectively, and entering and leaving the right side of the chain with rates $\delta$ and $\beta$, respectively. (Bottom) The model can be used to study protein “traffic” along a filament. In the case shown here, $q$, $\gamma$ and $\delta$ are equal to zero.