The article presents information on Polymerase gene (POLG) and its importance in mitochondrial diseases. It is stated that mitochondria has their own genome which is composed of a 16.5 kb circular double-stranded DNA molecule (mtDNA) and the first disease-related mutations in POLG1 were identified and reported in 2001. Some diseases influenced by POLG gene are Alpers' syndrome and a complex multisystem disease SANDO.