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Z-67 — also known as Sullivan's gas or the imbecile's gas (TV: The Zygon Inversion) — was a nerve gas that unravelled the DNA of Zygons. (TV: The Zygon Invasion) Naval surgeon Sullivan, (TV: The Zygon Inversion) working for UNIT, developed it after an attempted Zygon invasion, working on captured Zygons at Porton Down. (TV: The Zygon Invasion) The Doctor took it from UNIT, (TV: The Zygon Inversion) as well as the formula. The Twelfth Doctor pretended it could be released by the Osgood Box. (TV: The Zygon Inversion)