Some TMNT stuff really isn't for little kids.
Mutagen, also known as Retromutagen Ooze, is a mucilaginous substance that has served as a key plot device in nearly every medium of the Teenage Mutant Ninja Turtles. Any creature that is exposed to mutagen will rapidly increase in size, strength and intelligence (sometimes), and even gain superpowers. It is responsible for mutating, among others, Splinter, the four Ninja Turtles, and villains such as Alopex, Bebop, Rocksteady, Koya, and Bludgeon.
In this part of the franchise, the mutagen - here called Ur-Ooze ("Primordial Ooze") - is the primary matter of mutagen originally from the base of the planet Utrominon, home of the Utroms. It was responsible for both the preservation of the biosphere as well as for the operation of the Utrom technology (similar to the underground water of the Earth's biota). IDW's mutagen has special properties outside of creating mutants; it has healing and transformative properties on its own. For terrestrial organisms, it has tremendous regenerative abilities in the raw state, when it comes into contact with the blood of the recipient; in this way it can completely regenerate lost limbs and even organs and even reverse the aging process.
Takeshi Tatsuo was healed by it to the point where he was able to regrow a foot, and it kept him youthful when ingested. The mutagen was integral in the resurrection of Oroku Saki and Kitsune. Mutagen was also given to Arnold Jones to restore his long-lost muscle.
The Ooze is also the basis for a super-soldier serum, which the earthly geneticists Baxter Stockman had made - on behalf of the Utrom warlord General Krang - along with other projects. But a Ninja raid on Stockman's laboratory StockGen Laboratories, caused the only samples of the already finished preparations to be stolen and mingled in an accident in a unique cocktail of some experimental animals from the laboratory (Splinter and the Turtles) and an alley cat (Old Hob) were transformed into humanoid, intelligent mutants. Also among the victims were the Slash, Flyborg, Herman, Pete, Mondo Gecko, the Rock Soldiers Bebop, Rocksteady, Alopex, Koya, and Bludgeon.