Since our results showed that phosphorylation of ER-α Ser167, but not ER-α Ser118, was predictive of response to endocrine therapy, they suggest that, in breast cancer, phosphorylation of ER-α Ser118 occurs frequently without estradiol, whereas phosphorylation of ER-α Ser167 may occur frequently in response to estradiol binding. This evidence concerns the gene ESR1 and breast carcinoma.