These were patients with a mainly neurological presentation including 2 patients with PDHA1 deficiency, a patient with a severe encephalopathy and mutations in the GFM1 gene [27], a patient with NARP (neuropathy, ataxia and retinitis pigmentosa) and a patient with mutations in OPA1. The reason of these normal values is unclear and requires further investigation, but it may be explained, in the case of mitochondrial DNA mutations, because of the phenomenon of heteroplasmy, or in the case of nuclear DNA mutations, by the tissue specificity of the phenotype. Here, NPTX2 is linked to retinitis pigmentosa.