Valve has updated Left 4 Dead 2 earlier today via Steam and like clockwork the zombie co-op shooter sequel has a new Mutation gameplay mode that will be used for this week only. It's called Follow The Lifer Liter. Basically its a variant of Scavenge except that instead of fuel cans all over the level there are only one group of cans that appear in a specific location.The good news: you and your team of survivors know right where to go to get the cans. The bad news: All of the infected know where the gas cans will be too.

In related news, Valve polled its fans to ask whether to keep the Realism Versus mode as a permanent. gameplay mode and 68 percent said, "Yes." Valve says that they will announce whether Realism Versus mode will indeed be a permanent mode next week.