Inclusion Criteria:  Adult women (≥ 18 years of age) with proven initial diagnosis of breast cancer with evidence of loco-regional recurrent or metastatic disease not amenable to resection or radiation therapy. Documentation of histologically or cytologically confirmed diagnosis of breast cancer with IHC of estrogen receptor (ER) expression > 1% and/or progesterone receptor (PR) expression >1 % breast cancer based on local laboratory results. Scoring of 0 or 1+ for HER2 protein expression by a validated immunohistochemistry assay or +1/+2 with negative HER2 amplification FISH/ISH ratio lower than 1.8 or HER2 gene copy less than 4.0. Eligible subjects must have undergone a treatment with Palbociclib for at least 3 months. Measurable or evaluable disease as defined per modified Response Evaluation Criteria in Solid Tumours (mRECIST) V1.1 criterion (at least 2 entries). Premenopausal or postmenopausal status.  6.1 Patients who are not postmenopausal must have undergone a treatment with LHRH agonist.  6.2 Postmenopausal status is defined as:  prior bilateral surgical oophorectomy, or spontaneous cessation of regular menses for at least 12 consecutive months in case of doubt serum estradiol <20 umol/l and follicle stimulating hormone (FSH) levels >15 IU/L.  Exclusion Criteria:  Subjects with advanced, symptomatic, visceral spread, such as patients with massive uncontrolled effusions (pleural, pericardial, peritoneal), pulmonary lymphangitis, and over 50% liver involvement). Palbociclib treatment as part of a clinical trial or prescription prior to market approval (Nov 2016).