With the release of first-person puzzler Portal 2 this week, Valve has revealed that the original Portal has sold nearly four million copies worldwide.
However, this total does not include digital sales via its Steam platform, reports news site GameSpot.
Portal was sold as part of The Orange Box on Xbox 360 and PlayStation 3 along with Half-Life 2, Half-Life 2: Episode One, Half-Life 2: Episode Two and Team Fortress 2.
It is also available as a stand-alone retail release for PC, and as a digital download for Xbox Live Arcade, with the title Portal: Still Alive.
Sequel Portal 2has received critical acclaim, with a Metacritic rating of 95 out of 100, and reviews describing a sequel that is "significantly bigger and more robust in virtually every meaningful way, but without feeling any less dense."
It is also the first title to feature PC-derived Steam functionality on a home console -- gamers who purchase the title for PlayStation 3 will also be able to unlock a Steam Play copy of the game (i.e. the full PC/Mac version) for free by linking their PlayStation Network accounts with Steam.