all 4400 have a fictional neurotransmitter, promicin, in their brains, which gives them their powers. The government, afraid of what this large group would do with such power, had secretly been dosing all of the 4400 with a promicin inhibitor, which had worked on most, but not all, of the 4400. One of the inhibitor's side effects is a potentially fatal immune deficiency. The inhibitor is ultimately removed from the 4400 by a dose of promicin extracted by Kevin Burkhoff from the blood of the infant Isabelle, who was never given the inhibitor. At the beginning of the third season, the Nova Group, a terrorist faction made up of 4400s, emerges. Originally formed as a "defensive" group in the aftermath of the promicin-inhibitor scandal, the Nova Group eventually carries out numerous terrorist attacks against the government and NTAC. The group is responsible for many terrorist attacks including the assassination of the men involved with the promicin-inhibitor conspiracy; the attempted assassination of Ryland; the framing of Baldwin for murder; and the driving of another person to insanity. During the third and fourth season, it is revealed that only a certain faction from the future wants to see history changed. Another faction, which prefers the status quo, opposes the 4400, and has sent their own operatives, including Isabelle Tyler and "the Marked", into the past. The exact motives of both factions were not revealed. Eventually, Jordan Collier, a returnee who declares himself the savior of humanity, makes promicin shots available to the general public. However, only half of the human population can actually tolerate promicin, and thus develop superhuman abilities, while the other half die upon taking the shot. Although the government outlaws promicin use, thousands of previously ordinary people have developed superhuman abilities, severely complicating NTAC's task. Collier later annexes a part of Seattle and transforms it into "Promise City", a self-proclaimed paradise open to all people with superhuman abilities. The US government attempts to reclaim Promise City but meets with little success. At the conclusion of the series, Danny Farell's uncontrolled ability exposed some of the residents of Seattle to promicin, resulting in about 9,000 deaths and as many newly empowered humans, while at the same time forcing the remains of NTAC (now themselves mostly promicin-positive) to ask Jordan Collier and his followers (as the only group immune to the 50% chance of death from exposure) to become the de