Promicin is a neurotransmitter that is found in the brains of all 4400s. It was at first suppressed by NTAC using the promicin inhibitor while the 4400s were in quarantine, but once they were taken off the inhibitor, the promicin allowed them to develop abilities.
Promicin was first discovered by Dr. Max Hudson on October 11, 2004, eight weeks after The 4400 returned. He first found it inside Orson Bailey and other 4400s still in quarantine. Promicin was considered the fifth neurotransmitter allowing The 4400 and later Promicin-Positives to develop abilities.
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 effect are unpredictable, potentially giving any ability. However, as part of a government conspiracy, every 4400 was regularly injected with a promicin-inhibitor, suppressing their potential new abilities.
Since the government ceased injecting the promicin inhibitor, every 4400 has developed 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 have in turn used to create at least one group of soldiers with 4400 abilities, as demonstrated by the government's attack on Promise City. Jordan Collier stole the government's supply, and has been distributing it via human couriers to anyone who wants the shot.
An injection of promicin has a 50/50 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. Kevin 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's 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's essentially a bundle of wires that connects the two hemispheres and it's usually slightly larger in lefties than it is in righties. That's 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.
Starzl Mutation Edit
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 weren't 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 replicable, 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 counter-act the promicin inhibitor, Dr. Kevin Burkhoff developed a serum containing Isabelle's pure promicin. This serum neutralized the charge so the inhibitor was not able to cross membranes and could be flushed out of the body.