Results from a EBCTCG metanalysis in postmenopausal patients, showed that 5 years of aromatase inhibitor (AI) treatment significantly reduced recurrence rates by about 30% when compared to tamoxifen (RR 0.70, 95% CI 0.64–0.77) and led to a 10-year BC mortality reduction of about 15% (RR 0.86, 95% CI 0.80–0.94). This evidence concerns the gene CYP19A1 and breast cancer.