The Mutations are week-long events that offer new ways to play the game, such as this week's "Realism Versus" Mutation and the upcoming campaign-only "Chainsaw Massacre," which will give players chainsaws with unlimited gas--and nothing else.
Only one Mutation will be available at any given time, changing weekly. A monthly poll will allow players to vote on which Mutation they would like to return the following week.
Bringing three Left 4 Dead survivors together with their L4D2 counterparts in a new campaign along with new game modes, weapons and the 'Fallen Survivor' infected, The Passing costs $7 (560 Microsoft Points) on Xbox 360 but will be free on PC.
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