Marrow

As a child, Sarah was one of the underground-dwelling Morlocks. When Callisto, the Morlocks' leader, kidnapped the X-Man Angel to be her husband, Sarah came across the captive Angel. Sarah believed Angel was beautiful. He would become an idol of worship for her.

When the Marauders massacred the Morlock community, a young Sarah was 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 former Morlock leader Callisto cared for Marrow during this period and displayed a maternal attitude towards the young mutant. 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.

Time passed quickly in that harsh dimension. 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, including an attack on a Lila Cheney concert that brought them into battle with Cable, Callisto tried to hold back Marrow's more violent ways.

Marrow was next seen as a spokesman of a band of Morlocks after M-Day. She gave an interview to Sally Floyd for her ex-mutant diaries. Marrow remained underground to protect and give hope to the few mutants that remained and those that feared going to the surface to live normal lives. Marrow's interview was heavy with subtext towards the upcoming Superhero Registration Act. Marrow told Sally Floyd that the louder Congressman Sykes called for mutant registration, the easier it it would be for him to hide the truth. Americans were already labeled and registered a hundred different ways, such as social security numbers and credit cards. Such indignities afforded the privilege of police and ambulance services. Spider-Man explained that he trusted Sally Floyd because of this interview with Marrow.

Marrow was eventually welcomed to the new mutant island of Krakoa, created by Xavier, Magneto, and Moira X. She entered through the teleportation gateway alongside other villainous and fractious mutants, who had been invited to join the nation in order to heal mutantdom and start over as a whole species together.