U-Men

Belief System

The U-Men's belief system around the teaching of the fictional John Sublime. They believe the world is imperfect and must not be exposed to "fallen Earth". As such, they wear special containment suits (although they have to pay for the suit themselves) and only eat special "clean" food.
The true insanity of their religion is that they are humans that harvest the organs of mutants and implant the organs in themselves to get super powers. However, this process can kill them if the organ aren't transplanted right. They believe this turns them into a "Third Species" ("Homo perfectus" as Sublime calls them) and will steal the mutant organs of dead U-Men. They use special weapons (such as bullets and razors) to get said organs.
U-Men work in teams. Each squad has a team leader and his/her lackeys. These team often go on mission to collect organs.
Appearances
We first saw the U-Men as a group of them hunt down Angel Salvadore (who has fly-like powers). They are about to dissect her until Wolverine attacked and get killed. As such, more U-Men attack the X-Men's mansion only for Jean Grey and several students defeat them sending them running away in fear.
Next time we see them, Xorn was taking a class on field trip. Xorn gets distracted and left the group. A single U-Man got beaten by the students. The students discovered Xorn "slain" other attacking U-Men.
Kid Omega and his gang got high on a drug called Kick. Drunk, they decided to attack an U-Men headquarter. They end up killing several U-Men. Why are we suppose to treat them as threat?
A single U-Man was attacked by mutants on Planet X.
References:

• http://en.wikipedia.org/wiki/U-Men_(comics)