Mechanisms regulating mitochondrial DNA (mtDNA) amount according to developmental stage and tissue origin are still unknown. Recent works have suggested the role of epigenetic modification at a CpG island located in POLG gene, encoding the polymerase responsible for mtDNA synthesis. We found that this CpG island is highly methylated in human tissues, whatever developmental stages (after 12 weeks) or tissue energy-demand. This methylation level is not modified by the presence of genetic mutations that can alter the mtDNA amount (mtDNA or inactivating POLG mutations). Mechanisms involved in mtDNA amount control in human differenciated cells remain to be identified. (By Dr. Julie STEFFANN, http://jmg.bmj.com/content/early/2017/01/09/jmedgenet-2016-104335 )