Two Sega published games in the Aliens franchise, Aliens: Colonial Marines and Alien vs. Predator, have been delisted from Steam today.
The removal of both games at the end of the year may indicate that Sega’s license to sell these particular Aliens related products could be at an end. Their recently published horror game Alien: Isolation is still available for purchase and has not been affected. Activision pulled all of its licensed Marvel games from Steam, XBLA and PSN before the start of 2014 after their licensing agreement expired, making this seem like the most plausible scenario for the sudden delisting.
Aliens: Colonial Marines and Alien vs. Predator are both still available for purchase on Amazon, the Xbox Marketplace and the PlayStation Store as of writing. Green Man Gaming, the popular digital game retailer, currently has a listing for Aliens: Colonial Marines, but no longer has a listing for Alien vs. Predator.
Steam regularly has had games removed and returned in short order, so this may just be a brief intermission in both games being provided for sale on PC. Take the possibility of both games no longer being available on the platform with a grain of salt, but if you really have to own either before the new year you should probably snatch them up while you still can on a service like Amazon.