Jennifer Walters, the meek and mousy lawyer cousin of Bruce Banner, was the daughter of Sheriff Morris Walters. Agents of Nicholas Trask, a crime boss who had crossed paths with her father, shot and seriously wounded her on a day when her cousin was in town. Bruce gave her a blood transfusion, as no other donors of her blood type were available; the radioactive blood mutated Jennifer as it had her cousin, transforming her into the She-Hulk.

The younger daughter of history professor John Grey and his wife Elaine, Jean Grey was 10 years old when her mutant telepathic powers first manifested after experiencing the emotions of a dying friend. Her parents took her to be treated by Professor Charles Xavier. While Xavier treated Jean he also used her to fine tune his Cerebro machine. When Xavier introduced young Jean to the astral plane a part of her mind manifested as a Phoenix raptor and touched the mind of Scott Summers in the orphanage. Later, Xavier erected psychic shields in Jean’s mind to prevent her from using her telepathic powers until she was mature enough to control them.

The full details of Sarah’s early history remains unrevealed prior to her joining the underground community of Morlocks. She was a young child living with the Morlocks when they were targeted by the agents of the evil Mister Sinister, the Marauders, who began massacring the community. Although the X-Men managed to thwart the Marauders, scores of mutants were killed. Sarah was one of the few who survived, as she was rescued by Gambit (the leader of the Marauders to the tunnels.)

Little is known about Mystique's past due to the fact that she can eliminate the outward signs of aging with her shape-changing power, and it's not known exactly how old she is. She apparently learned to use her shape-changing power at a very early age, for there is no evidence known to the public or the government that Raven Darkholme ever looked like anything but a normal human being.

Before becoming the Fantastic Four, the group were scientists successfully launched into space, but intense cosmic rays penetrated the ship's shielding and irradiated the crew. On landing, they discovered they had mutated into superhumans, and Sue became the Invisible Girl. Sue's relatively static invisibility power made her seem like a weak link in the team at first, but she soon discovered that she also had the ability to project and manipulate invisible force fields, making her a more effective combatant.

Emma Frost was born into a wealthy old Boston mercantile family that arrived from England in the 1600s. The second of three daughters, Emma also had an older brother named Christian who had turned to substance abuse as an escape from the drudgery of his life. Unknown to their family or themselves, the three Frost sisters were mutants whose telepathic abilities matured upon reaching puberty.

Elizabeth "Betsy" Braddock was born in England. Raised in the small town of Maldon, Essex, Betsy was Sir James Braddock's second child; she, her twin brother Brian, and elder brother Jamie had a very privileged life. By the time their parents died, Betsy had become a charter pilot. After she and Jamie were taken hostage by the Red Skull's agents and freed by Captain America and Captain Britain, Betsy learned the latter was her brother Brian. At this time Betsy began to develop precognitive powers. She dyed her hair purple and took up modeling, while her powers grew to include telepathy.

Born at the Wundagore base of the High Evolutionary, in proximity to the eldritch energies of the Elder God Chthon; Wanda and her brother Pietro's mother, Magda, fled Wundagore shortly after their birth. Although not known at the time, Chthon formed a mystic bond with Wanda, who, it was later learned, was destined to serve the role of Nexus Being, a living focal point for the Earth dimension's mystical energy.

May and Ben Parker took full custody of their nephew Peter after his parents were killed in a plane crash.

Later on, Ben was killed in a burglary, leaving May and Peter all by themselves, as she continued to raise him as her own son.

Jessica Drew is the daughter of Jonathan Drew, research partner to Dr. Herbert Edgar Wyndham. When young Jessica suffered uranium poisoning in 1931, Dr. Drew was forced to inject her with his untested spider serum and seal her in a genetic accelerator. In stasis for decades, her aging greatly slowed, she awoke with no memories of her own past.

When a top-secret program attempted to recreate the original Weapon X experiment that involved the feral mutant Wolverine, they failed to secure a test subject that could survive the bonding of the virtually unbreakable metal Adamantium to their skeleton. Seeking to take the project in a new direction, the project's director, Doctor Martin Sutter, recruited renowned mutant geneticist Doctor Sarah Kinney and tasked her with creating a clone. Using the only available genetic sample from Weapon X, which was damaged, they were unable to salvage the Y chromosome after 22 attempts. Kinney then proposed they create a female clone, and though her request was initially denied, she still went ahead and produced a viable female subject, prompting Sutter to reconsider. Despite resistance from his protégé Doctor Zander Rice, whom he had raised after Rice's father was killed by a bestial Wolverine at the original Weapon X Project, Sutter allowed Kinney to proceed. As revenge for her insubordination, Rice forced Kinney to act as the surrogate mother for the clone, and she gave birth to "X-23."

Ororo Monroe is the descendant of an ancient line of African priestesses, all of whom have white hair, blue eyes, and the potential to wield magic. Her mother, N'dare, was an African princess who married American photojournalist David Monroe and moved with him to Manhattan, where Ororo was born.