Palbociclib in Combination With Adjuvant Endocrine Therapy for Hormone Receptor Positive, HER2 Negative Invasive Breast Cancer

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). All participants participating in the research study will receive the same dose of Palbociclib. While participating in the research study the participant will have the following tests and procedures: * Clinical Exams: The participant will have a physical exam on the first day of the month for first three months of therapy. After that, the participant will have a physical exam every other month until the end of the first year of therapy. After the first year, the participant will have a physical exam every 3 months during the second year of therapy. During the physical exam, the participant will be asked questions about their general health, questions about any problems that they may be experiencing, and any medications they are taking. * Blood tests: The participant will have blood drawn on the first and fifteenth days of the first three months of therapy. After that, the participant will have blood drawn every month for the remainder of the first year of treatment and the every other month until the end of therapy. These tests will be used to determine if the participant is having any side effects related to the study drug. * Electrocardiograms (EKG): The participant will have an EKG performed on the first day of the first three months of therapy. After that, the participant will have an EKG every other month until the end of the first year of therapy. After the first year of therapy, the participant will have an EKG every 3 months during the second year of therapy.