Breast cancer (BC) is the most common cancer in women worldwide. Early menarche and a later end to menopause have been shown to be associated with an increased risk of BC, as has oral contraceptive use and hormone replacement therapy. Furthermore, observational studies have shown that levels of sex steroid hormones in the blood are also associated with increased BC risk. Taken together, these results indicate that hormones associated with the ovarian cycle are associated with BC risk. The most reliable method to assess the association of these hormones with BC is through randomised controlled trials, however, these are long and costly, especially in the case of primary prevention trials of cancer. Therefore, we will be using a two-sample Mendelian randomization approach which uses genetic variants to proxy for hormone levels and investigate how variations in hormonal levels affects BC risk and survival.