For a complete understanding of biochemical reactions, information on complex stoichiometry is essential. However, measuring stoichiometry is experimentally challenging. Our lab has developed a FRET-based assay to study protein complex stoichiometry in vitro. This assay, also known as Job plot, is set up as a continuous variation of the molar ratio between the two species, kept at constant total concentration. The FRET (Fluorescence Resonance Energy Transfer) between the two fluorescently-labeled proteins is measured and the stoichiometry is inferred from the sample with highest FRET signal. This approach allows us to assess complex stoichiometry in solution.