Abstract: The present invention refers to an in vitro method for determining the efficacy of anti-HER2 therapy in the absence of chemotherapy in a patient with HER2+ breast cancer comprising the detection and/or quantification of the expression of HER2 in an isolated biological sample of the patient, either (1) before or (2) before and during the anti-HER2 therapy in the absence of chemotherapy treatment. The present invention also refers to the use of a gene expression product of HER2 as a as an in vitro marker for determining the efficacy of anti-HER2 therapy in the absence of chemotherapy in a patient with HER2+ breast cancer, or alternatively as an in vitro marker for deciding or recommending whether to initiate an alternative medical regime to anti-HER2 therapy without chemotherapy in a patient with HER2+ breast cancer.