Three families (6 affected individuals) with confirmed RNASEH1 mutations have previously been reported.2 An additional case was identified from variants of unknown significance published in exome-sequencing studies.7 These data were combined with the London-Oxford cohort and confirmed that PEO was a universal feature in patients with RNASEH1-related mitochondrial disease and that a substantial proportion of patients (57%) exhibited cerebellar dysfunction. Here, RNASEH1 is linked to mitochondrial disease.