Z-67

Z-67- also referred to by the Twelfth Doctor as 'the Imbecile's Gas'- was a nerve gas that unravelled the DNA of Zygons. A naval surgeon working for UNIT developed it after the attempted Zygon invasion of the 1970s or 80s, working on captured Zygons at Porton Down. "Somebody with a TARDIS" took it from UNIT, as well as the formula. (TV: The Zygon Invasion)