Formula X was a mutagen developed for military purposes. Samples were hidden in an underground bunker, which was discovered by prison warden Clement Darling. His experimentation with Formula X caused a mutant outbreak in Bayou City, Louisiana. The mutagen is a major plot element in The House of the Dead: Overkill.
At the height of the Cold War, Formula X was developed by military scientists to transform ordinary humans into powerful super-soldiers; the project, named Operation Overkill, failed. Samples of Formula X were locked in an underground bunker, which was under a prison run by warden Clement Darling.
In 1991, Clement discovered the bunker. With help from crime lord Papa Caesar and disabled scientist Jasper Guns, Clement sought using Formula X to cure his ailing mother. For unknown reasons, the mutagen was leaked from the facility, infecting the Bayou City populace. After physical and verbal abuse from Caesar (including threats to his sister Varla), Jasper injected himself with Formula X to exact revenge.
After discovering Caesar's link to Formula X, rookie AMS agent G and Detective Isaac Washington investigated the mutant threat. They followed Caesar to Clement's prison; Clement executed Caesar and claimed responsibility for the outbreak. In the bunker, Clement transplanted his mother's brain into Varla's body with Formula X; his mother mutated into a large creature, which "G" and Isaac defeated. The Darlings perished when Clement's dead man switch detonated the bunker.
Formula X was originally designed to be a super soldier formula, and lingering effects of this is the zombie's inhuman tolerance for pain, being able to function without limbs and, in some cases, without parts of their heads. True to it's purpose, the creatures it creates can be incredibly strong and durable, several of which can be completely immune to gunfire except for small, exposed organs, and can take quite an amount of punishment before going down. Others require severe trauma to be put down, and even then the chemical has proven to be able to revive the subject (with memories intact, should the subject be sentient).
The chemical can cross species, frequently infecting fish, birds, bats and even worms (the latter of which develop extremely sharp teeth and grow to enormous sizes). It provokes aggression in all subjects.