Terminal inverted repeats have been observed in the DNA of various eukaryotic transposons, even though their source remains unknown. Inverted repeats are principally found at the origins of replication of cell organism and organelles that range from phage plasmids, mitochondria, and eukaryotic viruses to mammalian cells. The replication origins of the phage G4 and other related phages comprise a segment of nearly 139 nucleotide bases that include three inverted repeats that are essential for replication priming.