Abstract: The present invention relates to a method for predicting the response of a patient diagnosed with breast cancer to treatment with an endocrine therapy drug, comprising the steps of: (a) measuring the binding of the estrogen receptor, obtained from a breast cancer tumor from said patient, generating a binding profile, said binding profile comprising the binding of the estrogen receptor from said patient on one or more co-regulator proteins or functional parts thereof in the presence and absence of one or more added phosphatases, (b) measuring the binding of the estrogen receptor, obtained from a breast cancer tumor from said patient, generating a binding profile, said binding profile comprising the binding of the estrogen receptor from said patient on one or more co-regulator proteins or functional parts thereof in the presence and absence of added estradiol, and, (c) predicting from said binding profiles the response of said patient to endocrine drug therapy.