Les Enfants Terribles

Les Enfants Terribles (The Terrible Children in English), was a project initiated in the late sixties by President Lyndon B. Johnson and finished by the early 1970s by Zero and carried out by Dr. Clark that aimed to produce clones of the legendary soldier and founding member of The Patriots, Big Boss. The name of the Project was presumably inspired by the the novel and 1950s film of the same name. Big Boss was sterilized by exposure to nuclear radiation (most likely caused during the nuclear testing at Bikini Atoll or by the explosions caused by the Davy Crocketts) and, as a result, natural reproduction was not possible.

The project was apparently considered during the early stages of the formation of The Patriots (it was used by Zero as an incentive for Ocelot to join the organization) but it was not initiated until later on. After a rift began to grow between Zero and Big Boss over the interpretation of The Boss's will, Zero took steps to ensure that The Patriots would retain their figurehead.

For more info on the history of the project see here.

The Twin Snakes
The initial phase of the project involved manipulation of Big Boss's genes to create genetically identical clones. An egg from Dr. Clark's Japanese assistant was fertilized with strains of Big Boss's DNA and transferred to the womb of another member of The Patriots, EVA, who acted as a surrogate mother. Of the eight embryos created this way, six were aborted, thus producing twin clones. Further work was done so that one clone recieved twin copies of each of Big Boss's dominant soldier genes, in an attempt to create a genetically superior soldier, while the other recieved his recessive traits, which were considered inferior for the project's goals.

The superior clone was designated Liquid Snake and the inferior clone Solid Snake. EVA's reference to Solid Snake as "David" and his prior knowledge of this name suggests that the twins were given full names at birth, possibly by EVA herself. However, Liquid Snake does not acknowledge his given name, and reacts angrily to Solid Snake suggesting he has one himself.

As a result of Big Boss's influence (Liquid states "he always told me I was inferior" during his speech atop Metal Gear REX), Liquid believed that he had inherited the inferior soldier genes of his father and thus believed that he was the inferior of the original twin clones. In spite of his genetic shortcomings, it was Solid Snake who defeated both Big Boss and Liquid Snake.

Solidus
The creation of the original clones was the last straw for Big Boss, who left The Patriots in disgust. As a result, Zero required a perfect clone of Big Boss to provide the DNA-based key to access the master AI (JD). Thus Solidus Snake (meaning neither solid nor liquid) was created as an exact clone of Big Boss, without doubling up the superior soldier genes as in Liquid. The egg-donor and surrogate mother used in this phase of the project are unknown, although it is possible that they used a different method of cloning that did not require either one of those. Though Solidus was later known as George Sears, it is unknown if this was his birth name or if he even had one.

Each clone had accelerated aging written into their genes in order to provide a fail-safe against the clones falling into enemy hands. In Solidus' case, this aging was apparently faster, thus providing the figurehead that The Patriots required (a US President in the image of Big Boss) in a relatively short amount of time.

FOXHOUND codenames
When David joined FOXHOUND, he was assigned the codename "Solid Snake". Since Big Boss was in charge of the unit at the time, he may have awarded him the codename in reference to his old FOX codename, Naked Snake. Another possibility is that the codename was part of the FOXHOUND ranking system in use at the time, wherein "Snake" may have represented soldiers with a skill level similar to Big Boss during his time with the FOX unit. A FOXHOUND codename also contained a personal prefix to distinguish between those with the same codename, thus David's clone designation of "Solid" was used. It is unclear why this was used or which individuals knew the meaning behind the designation.

Later, when Liquid joined FOXHOUND, he was awarded the codename "Liquid Snake". It is unclear if the ranking system was still in place (wherein his abilities would assumedly award him the same codename as his brother) or if Liquid chose the codename himself as the other members of FOXHOUND did at that time (he may have already been aware of his relationship to Solid Snake at this time).

Having never been a member of FOXHOUND, President George Sears was referred to as "Solidus Snake". However, he did at one point pose as Solid Snake, and the codename is seen as appropriate considering his lineage.

In other games

 * As she dies in Metal Gear Solid: Portable Ops, Elisa/Ursula had a precognitive vision of Naked Snake's future: that Big Boss will build his own Metal Gear, one of his sons will bring the world to ruin and another of his sons will save it. During this state, she explicitly uses the phrase "Les Enfants Terribles".
 * Les Enfants Terribles is a minor French terrorist group mentioned in the MSX2 version of Metal Gear 2: Solid Snake that the boss Running Man originally belonged to. The group is not mentioned by name in the PlayStation 2 version included in Metal Gear Solid 3: Subsistence, most likely to avoid any confusion. The Metal Gear Solid 4 Database, however, implies that both the project and Running Man's Terrorist Group's names were inspired by the 1950s film Les Enfants Terribles.
 * In Super Smash Bros. Brawl, when Snake fights another player also using Snake, the player can initiate a Codec conversation with Col. Campbell, who starts wondering, "Has the Les Enfants Terribles project really come this far?" Snake explains that his doppelganger has more than physical and genetic appearances that make them alike, it is also the fact that he has his fighting style as well, although it's not exact.

Trivia

 * In the novelization of Metal Gear Solid, Nixon asks if the birthing process of Solid and Liquid was their ninth attempt, to which Dr. Clark confirms, revealing that the previous eight attempts were failures until they corrected the genetic batch. It is unknown if EVA had given birth to any of the failed children, but it is confirmed that she was at least the mother of Solid Snake and Liquid Snake.
 * Going by this, it can be estimated that the clones that have been created would have been at least nineteen clones and at most sixty-seven clones (counting Solidus, and discounting any clones that were aborted to encourage fetal growth for Liquid Snake/Solid Snake and Solidus Snake).