Captain Marvel # 3 - Bitter Pills

In this issue, now made continuity headache inducing, Genis teams up with Adam-X AKA X-Treme, the character from the mid 90's who was allegedly intended to be revealed to be the 3rd Summers brother and has since been rendered irrelevant and best-left-unmentioned due to the creation of Vulcan.

Here, Genis and X-Treme team up to battle Eric the Red of the Sh'iar empire; thus making this one of 2 essential issues (the other being an issue of X-men with X-treme on the cover) that thoroughly sets up the clear intention that was in place at the time to show that X-treme was Cyclops and Havok's half-Sh'iar brother.