The Large deviations theory for stochastic processes has been mainly developed by Freidlin and Wentzell for Stochastic Differential Equations. I will give some insight about the tools that are used in this theory, in particular its analogy with Hamiltonian mechanics. I will then present a formal method for building the rate function of the Large deviations principle for a stochastic hybrid process used in cell biology for modeling gene expression dynamics, and how it can be used.