RRM2B and autosomal recessive progressive external ophthalmoplegia: Before this study, POLG1 had been the only established gene responsible for arPEO, while adPEO is a genetically heterogeneous disease, caused by rare variants in POLG1, POLG2, C10orf2, SLC25A4, OPA1 and RRM2B. The results of this study identifying the second responsible gene for arPEO indicate that arPEO is also a genetically heterogeneous disease, as is the case for adPEO.