Mutagen

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, and even gain superpowers. It is responsible for mutating, among others, Splinter and the four (or five or six) Ninja Turtles, and villain pairings such as Bebop & Rocksteady or Tokka & Rahzar.

1987 Series
The mutagen was depicted slightly differently in this cartoon; rather than causing its victims to increase in size, it caused them to transform into a hybrid of their original species and whatever creature they have most recently been in contact with. It was revealed in the episode "Invasion of the Punk Frogs" that the mutagen can be made mixing nyotrinaline with Taxon-9. In the episode "Enter: Mutagen Man", a third ingredient was mentioned - Bindex3. In the episode "Shredder & Splintered" it's acidic. It is also responsible for creating many of the show's mutants, such as Bebop, Rocksteady, the Punk Frogs, Leatherhead, Slash, Groundchuck, Dirtbag, and Mondo Gecko, to name but a few.

2003 Series
The mutagen was created by the Utroms while they were trying to create a machine that will allow them to escape Earth after they crash-landed on it. Disguised as T.C.R.I., they tried to create the machine secretly. However, during it's creation, the waste turned into a mutagen, with their attempts to dispose of it resulting in a cannister falling off the back of a truck and mutating the turtles and Splinter when it fell into a sewer and splashed all over them. Much like in the Mirage Comics, the mutagen causes explicit growth, both physically and intellectually. Among the victims were the turtles, Leatherhead and Splinter.

2012 Series
The mutagen is depicted as having similar properties to the version seen in the 1987 series; It blends the victim's genetics with that of whatever creature they have recently touched. It has a glowing blue coloration to it. Michelangelo often refers to it as mom in this cartoon due to the role it played in their creation, while Hamato Yoshi's human DNA was reponsible for the humanoid form they took. The mutagen also seems to merge organic substances and inorganic substances as seen by Spy-Roach and the same subject shows that it can also cause similar effects to the original comic version, increasing size and intelligence. However Parasitica shows that this isn't always the case.

As seen inThe Pulverizer Returns!,if a human has not come into contact with an animal or plant before using mutagen, their bodies will be molecularly broken down, turning them into a pile of acidic goop with only their internal organs still intact. Pulverizer also states that the mutagen burns a lot.

It has been stated that the mutagen was not intentionally created as a mutative agent; it apparently had different properties when it was created in the Kraang's home dimension, but the new laws of physics in this plane of existence altered its function.

Movies
As opposed to being labeled T.C.R.I. (Techno Cosmic Research Institute) in the Mirage comics, the canister in which the ooze is kept is written as "T.G.R.I." (Techno Global Research Industries), in compliance with the plot. In the first film, the mutagen is shown as having been the substance that mutated Donatello, Raphael, Michaelangelo, Leonardo, and Splinter from ordinary animals into anthropomorphic beings.

Mutagen was a much more integral part of the second movie, Teenage Mutant Ninja Turtles II: The Secret of the Ooze. Having identified TGRI as the source of the Ooze, the Foot Clan abducted Professor Jordan Perry and forced him to create a new batch of ooze, to create new warriors against the turtles. This mutagen was used to mutate a wolf cub and a baby snapping turtle into Rahzar and Tokka, respectively (Although Professor Perry was able to modify the new ooze to retard the subjects' intellectual capacities, both creatures being relatively infant-like where Splinter achieved human-level intellect fairly quickly and the turtles were only limited by their initial youth).

Near the end of the film, Shredder drank some mutagen to become Super Shredder. It is unknown what, besides his natural human form, animal species' attributes the Shredder gained during this time. Jordan Perry explains to the Turtles that the ooze that mutated the Turtles and Splinter was a mistake. In Perry's words, "an unknown mixture of discarded chemicals was accidentally exposed to a series of radiated waves", results in the creation of the ooze which has "remarkable, but dangerous mutagenic properties." Donatello was very disappointed and upset upon learning this, as he always hoped that there would be more to their origins than an accident, but Splinter encouraged him not to judge his worth based on his origins.

The canister was seen placed among the helmet of Shredder, the Time Scepter, and Yaotl's helmet in his trophy room in TMNT.

In Turtles Forever, there were small capsules of what appeared to be purple mutagen which Donatello took possession of from the 1988 Shredder. At least two members of the Purple Dragons were mutated by the mutagen. The first was Mohawk, who was mutated into a half-dog mutant, while Hun was mutated into a half-turtle which was similar to Slash. A great deal of mutagen was mass produced by Ch'rell and Karai. They used it to mutate human foot soldiers into a mutant army which they used along with upgraded robot 1987 Foot Soldiers in the foot's attack on New York in the 2003 universe.