Third Summers brother

The third Summers brother is a largely speculative plot point launched by 1993's X-Men (vol. 2) #23. In that issue Mister Sinister has a conversation with Cyclops and casually mentions Cyclops' brothers. Cyclops notes that it is odd to talk about his brothers in the plural, since as far as he knows he has just one brother (Alex Summers, also known as Havok).

While Sinister himself immediately claimed that it was just a common mistake of his, the speculation about the mysterious third Summers brother ran rampant among fandom. Gambit and Adam X were both believed to be the missing brother at some point.

Oddly enough, despite the naming of this occurence as "the third Summers brother", Sinister never says there are only three brothers.

Apocalypse
During his run on Cable, Robert Weinberg actually planned a rather complex series of circumstances that would have revealed that Apocalypse was in fact the third Summers brother all along, but Weinberg left the book before he could go along with his plan.

Gambit
The 12 issue, 3 part X-Men: The End series, which presented one possible future of the X-Men, established that Gambit was a genetically engineered mutant with his DNA spliced partially from Cyclops, thus making him a third brother in some respects. According to one review, "In strict theory, the idea is supposed to be that [the series takes] place in a possible future. Logically, that should mean that anything they reveal about the past also holds true for the mainstream Marvel Universe." In addition, the writer of the series was Chris Claremont, the creator of both Sinister and Gambit, although whether that gives greater credability to Gambit's status is debatable.

Vulcan
The 2006 series X-Men: Deadly Genesis established that the third Summers brother did indeed exist and was the previously unknown character known as Vulcan. It is not yet known whether Mister Sinister was even aware of Vulcan's existence, so the link between Vulcan and Mister Sinister's statement is still speculative. It is also pointed out by fans that Mr. Sinister's comment meant there was at least a third brother, he did not specify a number and therefore other brothers might also exist. In any case, the current version of the story would negate the scenario of Adam X being the son of D'Ken and Catherine Summers, as she was already pregnant when they met, and died before Adam X was born.

Adam X
Adam X is a currently underused character, who was thought to be the son of Catherine Summers and Shi'Ar Emperor D'Ken before the Deadly Genesis story line revealing Vulcan as third Summers brother and negating the possibility that Adam X was Catherine's son. While on Earth, Cyclops' grandfather crashes, and Adam uses his mutant ability to help the elder man, who remarks that Adam resembles his grandchildren.