Marrow

Marrow is one of the underground-dwelling Morlocks. During the Mutant Massacre that devastated the Morlock community, she witnessed the mauling of Angel by the Marauders. She was also personally saved by Gambit, who was not yet known by the X-Men. Years later, most of the surviving Morlocks (including Sarah) were apparently killed by Mikhail Rasputin, although they were actually brought to another dimension. There, Mikhail had established a citadel for himself on the top of a massive hill. The Social Darwinist philosophy of "survival of the fittest" was the only thing which governed the society. If someone could reach the summit of the hill, they were considered "fit" and worthy of being part of Gene Nation.

The Morlock leader Callisto cared for Marrow during this period and displayed a maternal attitude towards the young mutant, although the two are not known to be biologically related. When her powers fully manifested she was forced to "up the hill", becoming more savage and killing her opponents to survive. When she managed to reach the top, Mikhail considered her fit to join his elite fighting team Gene Nation, which eventually became a terrorist group whose mission was to punish humans for their hatred towards mutants.

In that harsh dimension, time passed quickly, and when Marrow and several other members of Gene Nation (of which she had become leader) returned to Earth, they discovered only a few years had passed during their absence. Marrow and the other members of Gene Nation began a homicidal terrorist campaign against surface-dwelling humans. After inconclusive battles with the X-Men and Generation X, Gene Nation was defeated by the X-Men. Marrow wired a time bomb to her own heart to force the X-Man Storm, who had herself led the Morlocks in the past, to either yield or kill her; Storm tore out Marrow's heart. However, due to Marrow's possession of two hearts to compensate for her random bone growth and superhuman regenerative abilities, she survived. Marrow eventually reunited with Callisto, and while they continued their terrorist activities, Callisto tried to hold back Marrow's more violent ways.

After a battle with Cable, Marrow and Callisto returned to seclusion. When Callisto was injured by a Prime Sentinel during the events of Operation: Zero Tolerance, she directed Marrow to seek the aid of the X-Men. After teaming up with Iceman and Cecilia Reyes, Marrow went to the Xavier Institute. She joined the X-Men for a time, coming under the mentorship of Wolverine and flirting with her teammate Cannonball.