The Promicin inhibitor was used by the government after the discovery of promicin in the 4400.
The promicin inhibitor was developed to suppress promicin within the 4400. It was kept hidden from the 4400 and members of NTAC and secretly given to the 4400 in doses when they went for check ups meaning they did not develop abilities. However, a few of the 4400 such as Maia still showed 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.
Since the inhibitor has stopped being given to the 4400 they have all developed abilities.