Amongst other mtDNA replication factors that have been associated with mitochondrial disease, dominant POLG2 variants cause PEO, ptosis, and myopathy, from infancy to late adulthood, often with additional neurological and non-neurological features [253–255], while recessive variants cause severe phenotypes akin to the POLG spectrum [255,256]. The gene discussed is POLG; the disease is ptosis.