The third expansion for "Half-Life" was "Decay". The game was again developed by Gearbox and published by Sierra. However, unlike previous games, "Decay" is only available with the PlayStation 2 version of "Half-Life". "Decay" is unique within the "Half-Life" series as the only cooperative game—two players must work together to progress through the game. "Decay" focuses on two of Freeman's colleagues, Gina Cross and Colette Green, as the two work with other scientists to counter the effects of the dimensional rift and ultimately attempt to close it.