SPOILERS FROM X-MEN DAYS OF FUTURES PAST
Recently it was announced that Beast appears human in parts of Days of Futures past because of a serum he created to control his mutation. Many people speculate that this is why it appears that Professor X is walking instead of being in a wheel chair.
Some people don't like this idea,saying that it takes away from the things that were built on in X-Men First Class, but this move by Brian Singer does fix some X-Men continuity issues.
X2
In X2 there is a cameo from Hank McCoy, aka Beast, in his human form. In X-men 3 Beast is in his blue form. This says that sometime in between X2 and X-Men 3, Hank McCoy transforms into his blue furry appearance. Now, at the end of X-Men: First Class Beast is already blue, which doesn't make sense if he is human in X2!
The serum idea fixes this continuity problem. Because Hank is ashamed of his blue mutation he hides in human form, and then sometime after X2 he realizes to be mutant and proud and abandons the serum.
X-Men 3 and X-Men Origins:Wolverine
In the opening scene of X-Men 3 we see a walking Patrick Stewart as Professor X confronting a young Jean Grey for the first time. This professor X is bald. In X-Men Origins:Wolverine we see a walking bald Professor Xavier trying to rescue young mutants. In X-Men:First Class Charles Xavier is paralyzed prior to his first encounter with Jean Grey, and still has hair. The idea of a serum concealing his wheel chair appearance would fix continuity issues with X-Men 3 and X-Men Origins:Wolverine. Another theory I've heard is that Hank constructs some sort of bionic walking devise for the Professor, which would also fix continuity problems.
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