This is just my theory

Now, due to Fox rights, they can not, in theory, say mutant. People have suggested numerous ideas for people

Some suggest magic based origins (Mostly for Scarlet Witch and by extension Quicksilver)

Others have suggested it will be lampshaded as classified

Me, I have a theory....and it starts with Marvels latest star....

In this universe, Hulk was a result of trying to recreate Captain America's Super Soldier Serum. Now, what else was an attempt at creating super soliders....

But the Ultimate Universe Mutants!?

Now, I know that people HATE that revelation, but it is a known fact that the Marvel Cinematic Universe takes a lot of cues from Ultimate Marvel. That gives an explanation that fits into the grander universe, and its done by Joss Whedon.

Lets give ol Joss a chance to make it work