The Essex Corporation is a mysterious organization led by Nathaniel Essex.
After Wolverine is released from his imprisonment by Nightcrawler, Cyclops and Jean Grey, he begins murdering several guards and is himself shot at, leaving trails of blood throughout the complex. During this commotion, Nightcrawler, Cyclops and Jean manage to rescue the other X-Men and Wolverine manages to escapes the Weapon X facility, disappearing into the woods. Sometime after the incident, a team of hazmat workers begins collecting the various mutant blood samples left behind and store them in a briefcase labeled "Essex Corp".
A branch of the Essex Corporation appears called the Essex House for Mutant Rehabilitation that fostered young mutants with sinister intentions in mind.
In the years that followed somehow the company Alkali-Transigen would get their hands on the blood sample from Wolverine this along with DNA from several other Mutants led the creation of the X-23 children in the mid-late 2010s who would be grown up in the Alkali Transigen Facility to be trained and raised as living weapons until their escape years later in the late 2020s.
- The Essex Corporation is an allusion to the X-Men comic book villain Mister Sinister, whose real name is Nathaniel Essex. In the context of the comic books, Sinister is a geneticist obsessed with creating the perfect mutant, his experiments leading to the creation of several, highly powerful mutants, such as: Madelyne Pryor, Cable and Nate Grey, all of whom are based off of Jean Grey's and Cyclops's genetic material.
- Mister Sinister also commanded a mutant hate group, The Marauders, whose members included: Sabretooth, Arclight, Riptide and Gambit.
- In the X-Men comics, Sinister was an ordinary human being living in Victorian-era England until he awoke Apocalypse from his eternal sleep. Interested in Essex's experiments on mutants, Apocalypse granted the geneticist mutant powers through genetic modifications. Sinister would later sever his ties with Apocalypse due to conflicting interests, creating Cable as a weapon against the ageless mutant. It is currently unknown if the Essex Corporation has had any contact with Apocalypse.
- It is possible that the Weapon X facility Ajax works in is part of the Essex Corporation. Ajax's work focuses on creating mutates which is Mr. Sinister's main goal.
- Notably, two Morlocks are present in the Deadpool film's Weapon X facility, Angel Dust and Marrow. This could indicate the Marauders brought in Morlock members to experiment on rather than massacre.
- It is also possible that Alkali-Transigen is a sub-faction of Essex Corp, as it’s stated to be owned by a larger company. Making them responsible for Project X-23, X-24, and the mutant suppression serum. In X-Men: Apocalypse the Essex Corp takes the samples of Wolverine's DNA from Weapon X that would eventually be used to create X-23.
- The Essex House for Mutant Rehabilitation is a possible subsidiary of Essex Corp.