Interestingly, in a study by Quinzii et al. [110], fibroblasts with COQ9 and PDSS2 mutations were found to have the lowest level of residual CoQ10 status (18% and 22% of control levels, respectively), whereas fibroblasts harbouring a COQ2 mutation had an intermediate level of CoQ10 deficiency (36–43% of control levels) and showed the highest level of oxidative stress generation. This evidence concerns the gene COQ9 and coenzyme Q10 deficiency.