The finding by an international team of researchers also provides strong support for the hypothesis that a small number of cells, called cancer stem cells, fuel a tumor's growth.

In the study led by the University of Michigan, researchers found cells from normal and cancerous breast tissue that had high levels of the enzyme aldehyde dehydrogenase activity, or ALDH, acted as breast stem cells. The scientists also found tissue expressing the specific form ALDH1 had the worst outcomes, suggesting the easily detected marker could be used to assess prognosis.

"This study is a big step because it provides a marker that's easy to use in both normal and cancer cells," said the study's senior author, Dr. Gabriela Dontu, a University of Michigan research assistant professor.

The researchers said their findings might also have implications for other types of cancer.

The study, which included scientists from South Korea and France, appeared in the November issue of the journal Cell Stem Cell.