There are many factors that influence whether someone will get breast cancer. These include things that cannot be changed such as family history, breast density or hormonal factors like age at menopause. They also include lifestyle factors that can be changed such as alcohol intake, smoking, physical activity, BMI and oral contraceptive use. For many of these lifestyle factors there are currently no methods to accurately measure the accumulation of these exposures over an individual’s lifetime. We believe that we have identified a method to accurately measure accumulated exposures in a blood sample, to quantify breast cancer risk associated with these exposures over their lifetime. We will start by developing a molecular measure of hormonal oestrogen exposures based on DNA methylation data from 2500 women from several international cohorts and including nested breast cancer case control studies. We hope to validate this measure of cumulative oestrogen exposure associated with DNA methylation in additional cohort studies such as ALSPAC. We will validate these measurements associated with cumulative oestrogen exposure in blood samples from 1200 women using an alternative methodology. We will also investigate how other lifestyle factors, which are known breast cancer risk factors, might influence this oestrogen exposure measurement. This study may provide a new way in which to measure exposures that will help in understanding a women’s risk of developing breast cancer. Future work will then test this measurement as method of measuring breast cancer risk.