Scientists have discovered what they claim is the first direct link between breast cancer in younger women and the sex hormone oestrogen.
An alteration in a gene involved with the breakdown of oestrogen is associated with a 9% reduction in the risk of breast cancer in women aged under 51, according to their research.
The report's senior author, Dr Olivia Fletcher from the Institute of Cancer Research, said: "This is the first time anyone has found a DNA change that is directly associated both with hormone levels and breast cancer risk in younger women.
"Scientists have suspected this link exists, but no one has been able to prove it until now.
"This represents an important step forward in our understanding of the link between hormones and breast cancer. Ultimately, it may have implications for the way we monitor and treat breast cancer."
Researchers identified a DNA variant more common in women with lower levels of a particular oestrogen breakdown product.
After then examining over 10,000 breast cancer patients and 17,000 healthy women, they concluded that the DNA variant was associated with a 9% decrease in the risk of breast cancer in women up to 50 years old.
Co-author Professor Isabel Dos Santos Silva, from the London School of Hygiene and Tropical Medicine, said: "Previous studies have shown that levels of the sex hormone oestrogen in women who have gone through the menopause are linked to their risk of getting breast cancer later in life.
"However, it has been difficult to establish whether such a link exists in younger women, partly because oestrogen levels fluctuate markedly throughout a woman's menstrual cycle and are therefore rather difficult to measure accurately."
She said the report, published in the Journal of the National Cancer Institute, could improve the way young women with breast cancer are diagnosed and treated.