To the Editor: Primary coenzyme Q10 deficiency is considered to be the only treatable mitochondrial disorder, since patients have a response to oral coenzyme Q10 supplementation. The disease usually manifests with nephropathy and encephalomyopathy.1 It has been shown that oral coenzyme Q10 may stop the progression of encephalopathy, but no benefit from this therapy has been noted with respect to the evolution of renal disease associated with this deficiency.1,2 We now describe the results of long-term coenzyme Q10 supplementation in two patients with coenzyme Q10 deficiency caused by a homozygous missense mutation in the COQ2 gene.3,4 The clinical . . .