Enigma is conveniently located in the Heaven bar within Gangland.
This mutation grants:
Enigma is easy to reach and grants the highest cool bonus besides vampire. The immunity to being appraised can also be an advantage in certain situations. Also some, but not all mutations are covered up with enigma so people can't tell what you have by looking at you. If you can recognize the symptoms of different ailments, and you can keep track of broken limbs and such, you can still be treated without much hassle.
Being un-diagnosable (for lack of a better term) can be a major or minor disadvantage depending on your soaks, endurance, and luck; don't take this if your character is prone to brain damage! Not a good mutation for hypochondriacs, people prone to mental illness, or most people who can't make use of the cool. Not a good mutation for someone new to the game either, as they'll not be able to properly guess their ailments well enough. Radiation areas also suck, as cancer becomes nigh impossible to detect.
Your appearance when diagnosing or appraising essentially disappears, along with visible mutation effects disappearing from the looking and examining.