Mutant

Mutants are the millions of people across the Imperium of Man who have been deformed from the baseline Human phenotypic state by genetic mutation. This mutation can be caused by environmental factors like ionising radiation or by exposure to the raw psychic power of Chaos.
These people are destined to become the outcasts and lower classes of Imperial society. They are shunned and are often hunted down and killed as the Imperium has little tolerance for any who deviate from the divine perfection of the baseline Human form in mind or body.
Mutation is actually considered a heretical crime against the Emperor of Mankind by Imperial authorities because of its connection to sympathy or worship of Chaos.
Human mutants are often terribly misshapen creatures, abominably deviating from the normal Human form. On the other hand, not all mutants are degenerate beasts; many only bear a few extraordinary traits setting them apart from the rest of Mankind.
While many mutants are born mutated as a result of their ancestors' exposure to mutagens like radiation or extreme planetary environments over many generations, exposure to the psychic energies of Chaos can also lead to rampant mutation.
Because those who secretly worship the Chaos Gods as Chaos Cultists are known to be granted mutational "gifts" by their dark masters, physical mutation can be an indicator of inner spiritual corruption by Chaos.
This is another reason that all mutants are considered suspect by the Imperium's authorities and are greeted as general social pariahs by almost everyone else across the Human-settled galaxy.
Navigators are also considered mutants, but may have been intentionally created through the use of genetic engineering in the distant past, during the Age of Technology to make it possible for Mankind to use the Warp-Drive effectively over long distances for interstellar travel.
Because they are absolutely vital to the Imperium's commerce and defence, they, alongside Imperial Sanctioned Psykers and certain species of Abhumans, are the only officially-tolerated form of mutants.

"While vile mutants still draw breath, there can be no peace. While obscene heretics' hearts still beat, there can be no respite. While faithless traitors still live, there can be no forgiveness." —Verse I of XXV of the Silver Skulls Space Marine Chapter's Catechism of Hate

Starting skills:
Acrobatics or Athletics, Awareness, Deceive or Intimidate, Forbidden Lore (Mutants), Survival
Starting Talents:
Weapon Training (Low-Tech, Solid Projectile)
Starting Equipment:
Shotgun (or stub revolver and great weapon), grapnel and line, heavy leathers, combat vest, 2 doses of stimm
Background Bonus:
Twisted Flesh: A Mutant character can always choose to fail any test associated with resisting malignancy or mutation. Whenever he would gain a malignancy, he may roll on Table 8–16: Mutations to gain a mutation instead. Starts with 10 corruption and rolls 5d10 to determine starting mutation. Has one of the following Traits: Amphibious, Dark-sight, Natural Weapons, Sonar Sense, Sturdy, Toxic (1), Unnatural Agility (1), Unnatural Strength (1), or Unnatural Toughness (1)
Background Aptitude:
Fieldcraft or Offense