The ALIQUOT study demonstrated that letrozole leads to a more complete suppression of plasma estradiol and estrone sulfate levels than anastrozole in a population of postmenopausal women with ER+ breast cancer (mean residual estradiol: 10% for anastrozole and 5.9% for letrozole; residual estrone sulfate levels; 4.6% for anastrozole and 2.0% for letrozole; p = 0.001) (80). Here, ESR1 is linked to breast cancer.