In several places of the Shipyard, the secret base of Harley Quinn, you can find posters of the "Cadmus Labs" which say "Building a brighter tomorrow".
In the comics, the Cadmus project was a scientific research center located outside of Metropolis and known for its achievements in genetic engineering and human cloning capabilities. Cadmus was involved in the creation of Superboy (mixing genetic samples of Lex Luthor and Superman) and Golden Guardian.
In the animated series Justice League Unlimited, Cadmus was a government project designed to take precautions against the League going rogue. They were responsible for creating Doomsday as a clone of Superman, and Terry McGinnis, the second Batman, using Bruce Wayne's DNA.
In the animated movie Batman Beyond: Return of the Joker, is hinted that Joker used Cadmus technology to overwrite his DNA into Tim Drake (the same Robin who appears in Arkham City and Harley Quinn's Revenge).
In Harley Quinn's Revenge, almost all the posters appear near to a Joker mannequin, maybe hinting at some cloning project for the Clown Prince of Crime - one of the possible ways to "resurrect" a dead character, together with Ra's al Ghul's Lazarus Pits.
Game available on: Sony PlayStation 3, Microsoft Xbox 360 & PC Video recorded on: Microsoft Xbox 360