One patient (6%) had a single mitochondrial DNA deletion, four (25%) had the m.3243A>G mutation, nine (56%) had POLG mutations, one (6%) had multiple mitochondrial DNA deletions in muscle with negative sequence analysis of POLG and C10orf2, and one (6%) had multiple mitochondrial DNA deletions in muscle with negative sequence analysis of POLG, C10orf2, RRM2B and SLC25A4. Except for diabetes in three patients with m.3243A>G, none of them had endocrine–metabolic disorders that could account for the peripheral neuropathy. Here, RRM2B is linked to diabetes mellitus.