The precise mechanisms underpinning the positive effects of physical activity on breast cancer risk and survival are unknown, although physical activity has been associated with a lower concentration of circulating estrogen in some studies [55,56], as well as lower circulating levels of the inflammatory mediators, C-reactive protein (CRP) [57-59] and interleukin-6 (IL-6) [60,61]. This evidence concerns the gene CRP and breast cancer.