Now, the Fantastic Four and X-Men do not interact too much in the comics, but there is one big thing that unifies them which came about in the 1990's if my memory of that era serves me correctly. Franklin Richards, the son of Reed Richards (Mr. Fantastic) and Sue Storm (the Invisible Woman) is in fact a mutant, and an incredibly powerful one. While he is just a young child he has abilities that are almost God-like when he doesn't try to block-off his powers lest he destroy reality. Now, the X-Men have of course recently established their own Nation for mutants and prefer that anyone who is a mutant goes and lives there, so that's created some friction between the X-Men and the Fantastic Four (touched upon briefly during the recent, "House of X/Powers of X," series where the Fantastic Four popped-by briefly) as while Franklin's parents may have powers, they ain't mutants--it was some weird space-stuff that gave them abilities along with Sue's brother, Johnny Storm (the Human Touch) and friend Benjamin Grimm (the Thing).