A drug used with letrozole to treat ER positive, HER2 negative breast cancer that has spread to other parts of the body. It is used in postmenopausal women whose disease has not been treated with hormone therapy. It is also being studied in the treatment of other types of cancer. Palbociclib blocks certain proteins, which may help keep cancer cells from growing. It is a type of cyclin-dependent kinase inhibitor. Also called Ibrance.