Mutagen, also known as Ooze, is a viscous substance, which is in virtually every medium and plays a key role in the origin of the Turtles. It changes the genetic structure of all organisms that come into contact with it, and so causes a mutation.
In the first Teenage Mutant Ninja Turtles GBA game based on the 2003 TV series, Mutagen canisters are used as health items.