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Peer Review is a free DLC made for Portal 2 aimed at expanding upon the co-op part of the game.
It continues the story of ATLAS and P-body as they assist GLaDOS in finding a mysterious intruder in the facility through the Cooperative Testing Course "Art Therapy", and adds a challenge mode for both single-player and co-op maps, allowing a player to race certain maps, trying to place the least amount of portals in the shortest amount of time, as well as compare their scores with other users.