Though the Fantastic Four have yet to appear in the Marvel Cinematic Universe, audiences have seen the importance of family in super teams thanks to groups like The Avengers and the Guardians of the Galaxy. But what those teams lacked was a common denominator that brought them all together. In the case of the Fantastic Four, the storm of cosmic rays that gave them their powers was what defined their future.
Source：https://www.cbr.com/fantastic-four-reed-richards-saddest-trait/
