In an analysis stratified by invasive and in situ breast cancer (data not shown), results for invasive cancer (1,150 ER+ and 629 ER– cases) were similar to the overall findings: the ORs for invasive ER+ cancer were 1.14 (95% CI = 1.00 to 1.31) for ever use, 1.41 (95% CI = 1.10 to 1.81) for use within the previous 5 years, and 0.95 (0.69 to 1.30) for duration of 15 years or more; the corresponding estimates for invasive ER– cancer were 1.36 (95% CI = 1.14 to 1.62), 1.62 (95% CI = 1.20 to 2.20) and 1.59 (1.11 to 2.29), respectively. This evidence concerns the gene ESR1 and cancer.