The risk ratio for breast cancer mortality after being treated with an aromatase inhibitor compared with being treated with tamoxifen was 1·25 (99% CI 0·85–1·85) in years 0–4 of follow-up and 0·80 (0·54–1·19) in years 5–9 (figure 1C, appendix p 9). This evidence concerns the gene CYP19A1 and breast cancer.