Otoyol 21, abbreviated as O-21, a.k.a. Ankara-Tarsus Otoyolu, is a completed toll motorway in the Central Anatolia and Mediterranean regions in Turkey. Currently, it is connecting Otoyol 20 from Gölbaşı, Ankara with the Adana-Mersin Motorway O-51 at Tarsus in Mersin Province. Wikipedia