Results from the ATAC (arimidex, tamoxifen alone or in combination) trial and analysis of the randomized ABCSG‐6a trial, revealed that treatment with aromatase inhibitors anastrozole or letrozole showed reduced treatment efficacy in women with ER+ BC with higher BMI compared with women with a healthy range BMI [54, 57]. This evidence concerns the gene CYP19A1 and breast cancer.