Palbociclib is a drug that may stop cancer cells from growing. Palbociclib blocks activity of two closely related enzymes (proteins that help chemical reactions in the body occur), called Cyclin Dependent Kinases 4 and 6 (CDK 4/6). These proteins are part of a pathway, or a sequence of steps, which is known to promote cancer cell growth. Laboratory testing has shown that palbociclib may stop the growth of hormone receptor positive breast cancer. Palbociclib is FDA-approved as therapy for metastatic hormone receptor positive HER2 negative breast cancer in combination with endocrine therapy in the first line setting, and in combination with fulvestrant for pre-treated disease. It is not known whether cancers that have grown despite prior palbociclib treatment are still sensitive to palbociclib.