Portal 2 sales have reached 4 million across all platforms, the game's developer has revealed.
Valve team member David Sawyer revealed the milestone in a recent interview, before discussing the game's new puzzle editor.
Mr Sawyer said the game's "fiction", as well as its design, made incorporating a puzzle editor an "idea too well for us to ignore".
"After Portal 2 shipped, we started to experiment with how the editor would work, and that initial idea carried through to what ultimately became the Puzzle Maker," explained Mr Sawyer.
"The Puzzle Maker lets people try their hand at creating puzzles while also letting them discover, share, and easily play what they and other Portal 2 players are coming up with," he continued. "So the editor needed to let anyone pick it up and not just use it, but enjoy the process of using it."
The Perpetual Testing Initiative DLC is now available for download for Mac and PC, and allows gamers to quickly and easily create their own "test chambers" in the Aperture Science Enrichment Center via a drag-and-drop interface, which can them be shared.
By Gaetano Prestia