Several studies have indicated the potential for circulating cell-free mitochondrial DNA (ccf-mtDNA) to be altered in patients with neurodegenerative conditions such as Parkinson’s disease and Friedreich’s ataxia,48,49 and similarly to GDF15 and FGF21, ccf-mtDNA has been suggested to be a useful biomarker of mitochondrial disease.50 Therefore, we studied ccf-mtDNA in the serum in samples from 42 CMT patients and 42 healthy controls by quantitative PCR. Here, FGF21 is linked to Friedreich ataxia.