The orphan Sarah
belonged to the underground group of mutant refugees known as the Morlocks,
until they were decimated by an opposing mutant faction. Sarah grew up
hating humans and "normal looking" mutants, and came to lead the surviving
members of the Morlocks. Her mutant genes that caused uncontrolled bone
growth horribly disfigured her, and she was never able to survive in the
outside world. She battled the X-Men, and was nearly killed. At the urging
of Callisto, the former leader of the Morlocks, Marrow joined the X-Men.
Although there was considerable tension at first, the others came to trust
their former adversary.