Abstract: The present disclosure provides an in vitro cell based potency assay to determine the relative potency of a composition, including a pharmaceutical composition, comprising an mRNA encapsulated in a lipid nanoparticle (LNP) as compared to a reference sample. Also provided is a process for releasing or accepting a batch of a pharmaceutical composition comprising an mRNA encapsulated in an LNP using the in vitro cell based potency assay. The methods and processes described comprise (i) transfecting a population of cells with a test sample of the composition, (ii) transfecting a different population of cells with a reference sample of the pharmaceutical composition, wherein the cells in step (ii) are the same cell type as the cells in step (i); (iii) detecting the amount of expression of a polypeptide encoded by the mRNA in the transfected cells; and comparing the amount of expression, thereby determining the relative in vitro potency of the composition.