Fragmented genomic DNA or complementary DNA (cDNA) of interest is cloned into plasmid vectors. The cloning sites are flanked with adaptor sequences that contain restriction sites for endonucleases (discussed below). Inserts are ligated to the plasmid vectors and individual vectors are then transformed into E. coli making the PET library. PET sequences are obtained by purifying plasmid and digesting with specific endonuclease leaving two short sequences on the ends of the vectors. Under intramolecular (dilute) conditions, vectors are re-circularized and ligated, leaving only the ditags in the vector. The sequences unique to the clone are now paired together. Depending on the next-generation sequencing technique, PET sequences can be left singular, dimerized, or concatenated into long chains.