Mutations

"Each week one new Mutation is featured and the previous Mutation retired."

- Valve

Mutations is the name for a new set of gameplay modes that was released alongside the new campaign, The Passing in Left 4 Dead 2. The Mutations are special game modes that run for one week before it is retired, and replaced by a new Mutation each week as they become announced on the in-game Blog Post every Friday. Periodically, a poll will be conducted for players to vote for their favorite Mutation, which will return the following week as an Encore Mutation, meaning it will re-appear the next week (See below for Encore Mutations). It is only available to those who have downloaded The Passing.

Revealed Mutations
There are over 20 different Mutations planned, and playable on all Left 4 Dead 2 Campaigns.

So far, the following have been revealed on Valve's blog in the scheduled order along with their taglines:

Unrevealed Mutations
WARNING: Spoilers.

The following were found in a Steam-Group annoucement and have not been seen in a schedule as of yet. They can also be found amongst the game's script files. As they have not been announced by Valve yet, they may still be subject to changes or removal.

Encores
After every four or five weeks, Valve may announce an encore week in which the community vote for a Mutation to be replayed. Only recent Mutations that have been played may be voted for. The following encore weeks re-showcased the following Mutations:


 * Friday, May 21st (week 5) had Last Gnome on Earth return for another week.
 * Friday, June 25th (week 10) had Chainsaw Massacre reappear. On the Xbox 360, it led with 43% of the votes. However it was much more close on the PC. Chainsaw Massacre won by 1%, specifically 500 more votes, than the second most voted PC Mutation: Headshot!.
 * Friday, July 30th (week 15) had a split vote. For the PC, Versus Survival won by 6% more than Four Swordsmen. However, on the Xbox 360, Four Swordsmen won by 10% more than Versus Survival.