Mutagen (2003 TV series)

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 turtles, and villains Tokka & Rahzar in Turtles Forever.

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 its creation, the waste turned into a mutagen, with their attempts to dispose of it resulting in a canister 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.