To the Editor We read with interest the study by Mitsui and colleagues1 who found decreased levels of plasma coenzyme Q10 (CoQ10) in patients with multiple system atrophy (MSA) regardless of the COQ2 genotype. Mutations in CoQ2 (which encodes an essential enzyme in the biosynthetic pathway of CoQ10) have previously been shown to be associated with MSA.2 Mitsui et al1 found a lower plasma CoQ10 level in those with MSA compared with controls (95% CI, 0.10; range, 0.02-0.66) (P鈥�=鈥�.02). Owing to the small sample size, the reference level of plasma CoQ2 used in the comparison had a major influence in the conclusion.