Effective description of small non-equilibrium systems

We consider effective description for small non-equilibrium systems with an illustrative example of two interacting proteins, of which one is driven by an external force, while the other is trapped by a potential. For this system, we review several problems related to three descriptions: the mechanical description for the atoms, the Langevin description for the residues, and the coarse-grained Langevin description for the proteins. We emphasize that universal relations valid far from equilibrium play a significant role in the consideration of effective description.