Valve is opening up Left 4 Dead 2's Mutations, a set of modified gameplay modes, to further modification by the community with the release of the Extended Mutation System, now in beta.
According to an update on the official Left 4 Dead blog, EMS is a "vastly improved scripting system" that will allow modders and script authors to have greater control over how Left 4 Dead 2 can play. While stock Mutations allow relatively simple tweaks to existing game rules — for example, giving everyone an M60 with infinite ammo, or requiring headshots to take down common infected — the update will let mod authors "create deep custom experiences that can be played on any map or tailor create something for new or existing maps."
The Extended Mutation System beta launches with a new survival mode called Holdout and examples of gameplay tweaks that let Survivors build land mines and barricades, as well as purchase emergency ammunition.
Valve writes that the Extended Mutation System will eventually roll into the main game and the recently launched Left 4 Dead 2 Steam Workshop.