Some TMNT stuff really isn't for little kids.

In the Mirage continuity, mutagen is a mucilaginous substance. Any creature that is exposed to mutagen will rapidly increase in size, strength and intelligence. It is responsible for mutating, among others, Splinter, the four Turtles and Leatherhead.

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 the technology company T.C.R.I. in New York City in order to allow the funds acquired through their transactions to help them return home, 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. It has a transformation property that mutates animals into intelligent beings. The mutagen causes explicit growth, both physically and intellectually.