A phase III, randomized, double-blind, placebo-controlled clinical trial (MA.17R, NCT00754845) (Study Details Letrozole in Breast Cancer Who Have Received 5 Years of Aromatase Inhibitor Therapy ClinicalTrials.gov, 2023) evaluated the efficacy of 5-year extension use of letrozole. This evidence concerns the gene CYP19A1 and breast cancer.