A canister of mutagen being shown to the Joker
Mutagen, also commonly called "ooze," is a mucilaginous mutagenic substance used in the movie Batman vs. Teenage Mutant Ninja Turtles, which is responsible for the Ninja Turtles and Splinter being what they are. Its exact origins and function are not specified, but it is produced at the TCRI company. Batgirl mentions that she hates the name "ooze".
In addition to mutating animals into intelligent anthropomorphic beings, the mutagen can be used to mutate humans, either through injection or through external exposure. The results of human mutation do not appear to be in any way related to what animal the subject most recently came in contact with, as the League of Assassins and Foot Clan ninja mutated with it took on forms of animals such as a mutant elephant, a mutant wolf, a mutant lion and a mutant Tyrannosaurus rex. Non-animal mutant forms can also result, as seen with Poison Ivy as a mutant plant monster.
When mixed with Joker Venom, the mutagen drives its subjects insane as well as mutating them, as seen with Batman as an unhinged mutant bat.