Rhabdomyosarcoma, the most common soft-tissue malignancy in childhood, is an aggressive cancer of a muscle-type cell. Led by David Langenau, PhD, researchers in the laboratory of Leonard Zon, MD, director of the Stem Cell Research Program, have mimicked human disease by producing gene mutations that create the most common form of rhabdomyosarcoma in the zebrafish. They recently identified the skeletal muscle stem cell from which the sarcoma arises. “We used the zebrafish because it is possible to look directly inside the fish using fluorescent proteins which mark different cell populations,” explains Langenau. “It is then possible to identify which cells are the cancer stem cells.”