Colomer et al(9) identified an association between the presence of the rs4646 single-nucleotide polymorphism in the 3′ untranslated region of the aromatase gene and efficacy of the AI letrozole, in terms of complete response rate and time to progression in 67 HR+ (hormone-receptor positive) advanced breast cancer postmenopausal patients. This evidence concerns the gene CYP19A1 and breast cancer.