For example, bi-allelic pathogenic variants in POLG, the gene encoding the catalytic subunit of the mitochondrial DNA polymerase gamma (Polɣ), cause a diverse spectrum of clinical phenotypes ranging from paediatric myocerebrohepatomyopathy spectrum disorder, and Alpers’ syndrome, to adult-onset chronic progressive external ophthalmoplegia (CPEO) and ataxia neuropathy spectrum (ANS) [8,9]. Here, POLG is linked to External ophthalmoplegia.