CHICAGO  - U.S. researchers have found a chemical that can kill breast cancer stem cells -- a kind of master cancer cell that resists conventional treatment and may explain why many cancers grow back.

Finding ways to destroy these cells could make cancer far easier to cure.

"There is a lot of evidence to suggest now that these cells are responsible for many of the recurrences that are observed after treatment has stopped," Piyush Gupta of the Massachusetts Institute of Technology and the Broad Institute, whose study appears in the journal Cell, said in a telephone interview on Thursday.

The problem is that cancer stem cells are rare and difficult to study in the lab because they quickly change into other types of cells. And they are hard to kill.

"It wasn't clear it would be possible to find compounds that selectively kill cancer stem cells," Gupta said in a statement. "That's what we did."

To study the cells, Gupta's team first devised a method for stabilizing cancer stem cells in the lab and getting them to multiply. They then tested them against 16,000 natural and commercial chemical compounds to see which ones were able to kill the cancer stem cells specifically.

That turned up 32 contenders.

They narrowed down this list to a handful of chemicals, and tested these in the lab and in mice.