Unlike other endonucleases, the MmeI (type IIS) and EcoP15I (type III) restriction endonucleases cut downstream of their target binding sites. MmeI cuts 18/20 base pairs downstream and EcoP15I cuts 25/27 base pairs downstream. As these restriction enzymes bind at their target sequences located in the adaptors, they cut and release vectors that contain short sequences of the fragment or cDNA ligated to them, producing PETs.