Batman Arkham City expansion Harley Quinn's Revenge is no longer downloadable from the Games for Windows Live Store.
Arkham City developer Rocksteady Studios has announced it is looking into the unexplained matter.
"We are currently investigating the mysterious disappearance of Harley Quinn's Revenge from the G4WL store," Rocksteady Studios community manager Sarah Wellock wrote on Twitter. "Apologies for any inconvenience."
Steam users previously reported problems verifying the content, a process that requires Games for Windows Live's systems.
The expansion is currently missing from the Games for Windows Live page for Batman: Arkham City.
"Harley Quinn's Revenge doesn't offer much that you won't have seen before, but it's unexpectedly tart and pleasantly grim," Chris Donlan wrote in Eurogamer's 7/10 Batman: Arkham City - Harley Quinn's Revenge review.