Valve has released an Expanded Mutation System for Left 4 Dead 2 which will allow modders to create more complicated creations.

The system is currently in beta and is an improved scripting system that will allow modders “to go past modifying existing scripts and write custom script logic, spawn and control entities, and much much more.”

This improved script should enable new game modes–and other cool things–to be created by talented modders and will be released to the wider community after testing.