It's worth taking script leaks with a pinch of salt, but numerous prominent movie websites were approached by a member of the X-Men: Days of Future Past crew who was trying to sell them a copy of the script. They apparently provided proof by way of the first page of the script and also a description of the death of one mutant, and it wasn't long before this information made its way online. The mutant to die is apparently Colossus, who is killed in a fight with a Sentinel. It is of course possible that this is misinformation, but if they do kill off the cruelly under-served character, it's going to disappoint a lot of people: his role in the X-Men movies to date has been strangely minimal, despite him being a fan favourite character, and one who desperately needs a larger role. Hopefully he'll at least get a big "moment" before he dies, perhaps sacrificing himself to save others.