Palbociclib, the first oral CDK4/6 inhibitor, is an approved medicine indicated for the treatment of a kind of advanced/metastatic breast cancer (MBC), called hormone receptors positive (HR+)/ Human epidermal growth factor receptor 2 negative (HER2-) disease. Palbociclib is given orally in combination with hormonal therapies. The purpose of this study is to better understand how Palbociclib combination is used in real-life conditions and its clinical impact compared with hormonal therapy. The study will also evaluate how long patients take the different CDK 4/6 inhibitor drugs and whether using those drugs impacts the use of chemotherapy later. Male and female patients aged 18 years old or more presenting the following conditions will be selected for the study: * HR+/HER2- MBC * First treatment with Palbociclib, hormonal therapy, or other CDK4/6 inhibitors after MBC diagnosis The study will use data without personal identity, which were obtained from medical records in routine clinical practice.