The purpose of this neoadjuvant trial is to evaluate efficacy and toxicity of the cdk 4/6 inhibitor palbociclib when added to standard endocrine treatment. Initially, patients receive endocrine treatment for 4 weeks. In case of decrease of proliferation (Ki67) patients are then randomized between either continuous endocrine therapy (arm A) or the same treatment with addition of palbociclib (arm B). Patients with no change of proliferation are allocated to endocrine treatment + palbociclib without randomization (arm C). During the 12-weekly treatment period, clinical and radiological evaluations are performed repeatedly. Switch between the treatment arms A and B is allowed in case of lack of response or due to toxicity. A translational subprotocol is a mandatory part of the study protocol, except for use of PET-CT evaluations.