A chemical called salinomycin hit the target. It was 100 times more potent at killing breast cancer stem cells than the common chemotherapy drug called paclitaxel or Taxol.

Cancer stem cells treated with salinomycin were far less able to start breast cancers when injected into mice than cancer stem cells treated by paclitaxel. And the treatment also appeared to slow the growth of tumors in the mice.

Gupta said it is not clear if salinomycin will emerge as the best drug compound for killing breast cancer stem cells -- or that it will be safe to use in people with cancer.

But the study offers a new roadmap for drug companies to isolate and test compounds capable of killing the cells.

"We now have an approach that can be used very systematically to find such compounds," he said.

Ultimately, he said it may be possible to treat cancers with dual therapies that wipe out the bulk of tumor cells and the tumor-cell making machinery many conventional treatments leave behind.

Cancer is the No. 2 killer of Americans, with about 560,000 deaths annually, topped only by heart disease, according to the American Cancer Society.