RETN and breast cancer: We performed a MR analysis to evaluate the causal relevance of six previously reported circulating adipokines [adiponectin, hepatocyte growth factor (HGF), interleukin‐6, leptin receptor, plasminogen activator inhibitor‐1 and resistin] and CRP in risk of overall and oestrogen receptor‐stratified breast cancer in up to 122,977 cases and 105,974 controls of European ancestry.