The proteins are called epigenetic proteins and they cause "mammary gland stem cell" numbers to expand when exposed to progesterone.

The researchers — from the Princess Margaret Cancer Centre in Toronto, Canada — describe how they discovered this in a paper due to be published in the Journal of Cell Biology.

Their findings represent a step forward in the development of "chemoprevention" for those at high risk for breast cancer.

"Currently," says study senior author Rama Khokha, a senior scientist at Princess Margaret Cancer Centre and professor of biophysics at the University of Toronto, "there are no standard of care preventative interventions for women at high risk of breast cancer."

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