By the endosymbiosis theory, which gained acceptance around the 1970s, the mitochondrion, as a major energy factory of the cell, was previously a free-living prokaryote that invaded a eukaryotic cell. Under this theory, symbiotic organelles gradually transferred their genes to the eukaryotic genome, implying that mtDNA was gradually integrated into the nuclear genome. Despite the metabolic alterations and functional adaptations in the host eukaryotes, circular mitochondrial DNA is contained within the organelles. Containing 37 genes, mitochondrial DNA has an essential role in the production of necessary compounds, such as required enzymes for the proper function of mitochondria. Specifically, it has been suggested that certain genes (such as the genes for cytochrome oxidase subunits I and II) within the organelle are necessary to regulate redox balance throughout membrane-associated electron transport chains. These parts of the mitochondrial genome have been reported to be the most frequently employed. Mitochondria is not the only location within which the cell mtDNA, mitochondrial DNA, can be found; sometimes transfer of mitochondrial DNA from organelles to the nucleus can occur; the evidence of such translocation has been seen through the comparison of mitochondrial DNA sequence with the genome sequence of the counterparts. The integration and recombination of cytoplasmic mtDNA into the nuclear DNA is called Nuclear Mitochondrial DNA, which is abbreviated as NUMT.