Z-67

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