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. 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.

Unofficial Mutations
The following were included in Mutations Mod. They are user-made custom Mutations and it's unlikely that Valve puts them in official rotation. A lot of people believe these are officially planned Mutations, even though it is wrong.