After the screening procedures confirms that the participant is able to participate in the study. The participant will be given a dosing diary for each treatment cycle. Each treatment cycle lasts 28 days, during which time the participant will take Palbociclib once a day on days 1-21 of each 28 day cycle and the aromatase inhibitor that the participant is already taking once a day every day. The diary will also include special instructions for taking the study drug(s).