Project Helix was a breakthrough in next-generation molecular gene therapy created by Doctor Enzo Markovic. It refers to a new procedure whereby a patient's face and body are reshaped to identically resemble someone else.
Unlike traditional plastic surgery, Project Helix literally reshaped the subject's genetic code to alter their physical attributes, requiring use of an offsite computer server in order to cope with the sheer scale of data needed to map a person's entire genetic structure. Because of that, the procedure does not leave any obvious surgical scarring upon the recipients. It only works on people of a certain genetic disposition (presumably those that baseline matches those of the person to be copied by the recipient) and the recipient must be induced into a comatose state for several days while their cells regenerate. An early report stated that the procedure included a flaw in the iris to distinguish between the original and the subject, but this was revealed to be a lie to allow one of the first subjects of the procedure to infiltrate the CIA.
To date, it is unclear if Project Helix is reversible, as all those subjected to the procedure have been killed while disguised. Allison Dorren spent two years with the face she received from the procedure even when it was no longer necessary, but this may have been because nobody had the skill to reverse the procedure at the time, as it was another two years before Project Helix was used once again.
Enzo Markovic: He created the procedure and used it to turn himself into the double of CIA agent James Lennox; his equipment was destroyed and he was killed in a confrontation with Sydney Bristow and the original Lennox.
Allison Doren: An assassin, Allison was hired by Arvin Sloane and Julian Sark to infiltrate Sydney's life by replacing her friend Francie Calfo. She continued to wear Francie's face even two years after her cover was exposed, and was eventually killed by Francie's friend and would-be lover, Will Tippin.
Irina Derevko doppelgänger: A female agent of the Covenant whose real name wasn't revealed volunteered to be turned into the double of Irina Derevko. This woman was only described as being an ardent follower of Elena Derevko, and fully aware of the fact that she was being set up to die.
Anna Espinosa: She was transformed into a duplicate of Sydney Bristow as part of a complex plan to undermine and defeat Sydney; she was eventually killed by Sydney.