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|Plot element from the 4400|
|Function||Grants the user their 4400/Promicin power, usually one powerful ability|
|Specific traits and abilities||Fluorescent yellow/green liquid|
The human body produces neurotransmitters that control and regulate bodily functions. In The 4400, every 4400 produces a fictitious neurotransmitter called promicin that enables him or her to use parts of the cerebellum no human has previously used. This is the cause of the new abilities in each returnee. Promicin's behavior and effects are unpredictable, potentially giving any ability. However, as part of a government conspiracy, every 4400 is regularly injected with a promicin-inhibitor, suppressing their potential new abilities.
Since the government ceases injecting promicin inhibitor, every 4400 develops an ability.
Dr. Kevin Burkhoff devises a way to artificially create 4400 abilities through a series of promicin injections, using himself as a test subject, giving him regenerative abilities. However, the government finds out about this breakthrough and builds up its own stockpile of a much purer promicin (extracted from Isabelle Tyler), which they in turn use to create at least one group of soldiers with 4400 abilities, as demonstrated by the government's attack on Promise City. Jordan Collier steals the government's supply, and begins distributing it via human couriers to anyone who wants the shot.
An injection of promicin has a 50% chance of either killing the person taking the injection (by way of an aneurysm) or giving them a 4400 ability. In the fourth season, Dr. Burkhoff claims to have discovered how promicin interacts with the body, thus being able to predict whether a person will survive the shot or not. It is explained that nine out of ten left-handed people who take the shot survive. In the brain there is a small part called the corpus callosum. It is essentially a bundle of neural fibres that connects the two hemispheres and it is usually slightly larger in left-handed people than it is in right-handed people. That is why left-handed people survive; the size of that part of the brain helps determine whether or not a person will be able to integrate a new neurotransmitter. With further research, Dr. Burkhoff believes a simple CAT scan will be able to tell how anyone's body will react to a promicin shot.
There exists a substance that can eliminate all traces of promicin in the human body, effectively robbing the injected individual of any 4400 powers and in addition making this person allergic to promicin. The identity and makeup of this substance is not stated, and the only known source of it so far is from the future. A second method also exists for removing promicin from non-4400 humans, through the promicin-neutralizing ability of Jordan Collier.
There also appears to be a substance that can counteract the anti-promicin's effects as Tom Baldwin, during his time as a member of the "Marked", uses this substance to help Isabelle Tyler regain her powers by removing her promicin allergy. Like the anti-promicin, this substance appears to be from the future.
As discovered by Marco in the episode The Starzl Mutation; Seattle Presbyterian Hospital started using a flawed radiation machine to treat their cancer patients in 1969. The microswitches that controlled the machine were not functioning properly, so the doses were incorrect. This went on for just under five years. Once the hospital figured out what they were doing they tested everyone who had ever been exposed to it and in some cases the radiation induced a small mutation. This mutation was a slight alteration of the eleventh chromosome, named the "Starzl mutation" after the manufacturer of the faulty machine.
The mutation is hereditary, meaning people that got it passed it onto their children. The Starzl mutation is both harmless and conserved in those offspring; they live normal healthy lives. While the offspring of a returnee normally do not retain the ability to generate promicin, a second-hand report from John Shaffner (an ex-special ops soldier) suggested that the offspring of a 4400 and a Starzl mutant will be "promicin-positive", meaning they will have abilities. This is believed by Tom Baldwin and Diana Skouris to be the reason why the abductees were returned to the Seattle area - it is the only place they are likely to encounter individuals who possess the Starzl mutation and therefore the only place they are likely to produce children with 4400 abilities.
The promicin-inhibitor "piggybacks on glucose", after entering the brain through facilitated diffusion. It is a binding protein; that is, it binds itself to promicin in the body and neutralizes it. However, if there is no promicin for it to bind to, it remains in the body. Eventually, it builds up to toxic levels in the lymph nodes, damaging the immune system. The result is a chemically induced immunodeficiency.
To counteract the promicin inhibitor, Dr. Burkhoff develops a serum containing Isabelle's pure promicin. This serum neutralizes the charge so the inhibitor is not able to cross membranes and can be flushed out of the body.
Alternate Reality Game/Viral Marketing
NBC/Universal created several sites targeted at fictional members of the 4400 universe:
Subsequent to the cancellation of the series, these web sites were allowed to lapse and none of them are active any further. Most of them have now been parked.
More information about the campaign (non-fictional) can be found in an ADWeek article.