Milwaukee ? Scientists have pinpointed a key protein that causes cancerous cells to spread throughout the body, a finding that could enable doctors to better predict how aggressive breast tumors might be.

The authors say the findings suggest a relatively straightforward way to halt cancer progression in women.

“This is really an important advancement,” said Brian Volkman, an associate professor of biochemistry at Froedtert & Medical College of Wisconsin Cancer Center. He was not involved with the study.

“We’ve been looking for things that are not just correlative, but causative,” he said. “This is something that turns a cell from being normal into something that’s cancerous.”

Metastasis, or the spread of cancer from one part of the body to another, is the main cause of treatment failure and death in cancer patients. The American Cancer Society estimates there will be more than 182,000 new cases of breast cancer diagnosed in women nationwide and more than 40,000 deaths this year.

For the study, researchers from the Lawrence Berkeley National Laboratory at the University of California, Berkeley, looked for a protein called SATB1 in two dozen breast cells, finding it only in cancerous cells that had spread. The researchers then examined more than 2,000 human breast cancer tissue samples and found that patients with the highest levels of SATB1 had the shortest survival times. Patients whose tumor samples had no SATB1 generally lived longer.

“SATB1 is usually a good guy, but in this case, it becomes a crime boss, according to nature,” said Terumi Kohwi-Shigematsu, a senior scientist at the laboratory and author of the study.

In breast cancer, Kohwi-Shigematsu said, SATB1 works by altering the behavior of more than 1,000 genes within tumor cells, including HER2, which stimulates cell growth and is associated with aggressive breast cancers.