The a transfusion of Bruce Banner's blood. It won't be long before She-Hulk comes to the MCU, with Marvel planning a She-Hulk Disney+ series expected sometime in the next year.
Stan Lee created She-Hulk back in 1980, and she was actually introduced for business reasons rather than creative ones. CBS' The Incredible Hulk TV series had proved a tremendous success, and Marvel feared television execs would create a She-Hulk first. Marvel published their own She-Hulk first, a smart way of ensuring they were the ones who owned the rights to the character. Ironically, Jennifer Walters has proved something of a hit.
In 2014, writer Alan Davis wrote a new Savage Hulk miniseries that subtly changed She-Hulk's history - after a fashion. It saw an early X-Men team - Cyclops, Iceman, Marvel Girl, Havok, Beast, Polaris, and Angel - attempt to help Bruce Banner cure himself of the Hulk. Banner had developed advanced technology to manipulate his own mind, but it had failed, and so he had given it to Xavier so the Professor could use it to repel an alien invasion. Xavier believed the telepathic component was the only reason it failed, and he attempted to help. Unfortunately, something went badly wrong - and Jean Grey absorbed the Gamma mutation, becoming the first She-Hulk.
Gamma radiation has a different effect on every person. For Jean Grey, it unlocked the the Phoenix Force.
Jean certainly made an impressive She-Hulk, and she would have wielded the combined power of the Hulk and an Omega level mutant. In the end, though, the false reality was dispelled and Xavier realized he had been unsuccessful. The potential cure briefly backfired, granting the Hulk the telekinetic potential of Jean Grey and allowing him to level an entire mountain; the X-Men barely escaped with their lives, and they never tried to cure the Hulk again.
