Z-67
From Tardis Wiki, the free Doctor Who reference
Z-67 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. The Doctor took it from UNIT, as well as the formula. (TV: The Zygon Invasion)
