Around a third of patients with mitochondrial disorders develop peripheral neuropathy, but this is mostly mild or subclinical.1 However, mutations in genes involved in mitochondrial dynamics (MFN2, GDAP1),2, –, 6 in mtDNA replication and maintenance (POLG, TWNK, TYMP, MPV17)7, –, 10 or RC complex V MT-ATP6,11, –, 14 are known to cause peripheral neuropathy as the initial manifestation or dominant feature of mitochondrial disease. The gene discussed is TWNK; the disease is peripheral neuropathy.