Thus, our work in human skin fibroblasts with pharmacologically induced defects of ubiquinone biosynthesis have confirmed that increased ROS production contributes to the pathomechanism of CoQ10 deficiency associated with inhibition of COQ2 and that partial CoQ10 deficiency (40–50% residual) is associated with increased ROS production, hyperpolarization, and cell death compared to CoQ10 defiency that is mild (>50% of normal) or severe (<30% of normal) [2], [3]. The gene discussed is COQ2; the disease is coenzyme Q10 deficiency.