The m.3243A>G pathogenic variant in the mitochondrial DNA (mtDNA)-encoded MT-TL1 gene has been identified in 80% of MELAS syndrome patients.9,10 Identification of other pathogenic mtDNA variants11-13 and recessively inherited POLG variants14–16 have also emerged as rarer causes of MELAS. The gene discussed is POLG; the disease is MELAS syndrome.