Breast cancer stem cells exist in two different states and each state plays a role in how cancer spreads, a new study has revealed.

Study's senior author Max S. Wicha from University of Michigan Comprehensive Cancer Center said the lethal part of cancer is its metastasis so understanding how metastasis occurs is critical.

"We have evidence that cancer stem cells are responsible for metastasis - they are the seeds that mediate cancer's spread. Now we've discovered how the stem cells do this," Wicha said.

First, on the outside of the tumour, a type of stem cell exists in a state called the (EMT) state. These stem cells appear dormant but are very invasive and able to get into the bloodstream, where they travel to distant parts of the body.

Once there, the stem cells transition to a second state that displays the opposite characteristics, called the mesenchymal-epithelial transition state (MET). These cells are capable of growing and making copies of themselves, producing new tumours.

The study looked specifically at breast cancer stem cells but the researchers believe the findings likely have implications for other cancer types as well.

The study was published in the journal of Stem Cell Reports.