  For continuous-time Markov chains and open unimolecular chemical reaction
networks, we prove that any two stationary currents are linearly related upon
perturbations of a single edge's transition rates, arbitrarily far from
equilibrium. We extend the result to non-stationary currents in the frequency
domain, provide and discuss an explicit expression for the current-current
susceptibility in terms of the network topology, and discuss possible
generalizations. In practical scenarios, the mutual linearity relation has
predictive power and can be used as a tool for inference or model
proof-testing.
