Mutagene

A mutagene is a gene that appears within a Smurf's body in the EMPATH: The Luckiest Smurf story series that controls the natural physical form of a Smurf. When the gene is manipulated by any kind of outside force, the physical form of a Smurf changes to whatever form the outside force makes it change into. Empath eventually discovered the mutagene during his visits to the Smurf Village after experiencing a physical transformation.