Palbociclib (Ibrance, Pal) is a selective inhibitor of cyclin-dependent kinase 4/6 (CDK4/6) approved by the Food and Drug Administration (FDA) in 2015 for the treatment of hormone receptor-positive (HR+)/HER2 negative (HER2−) breast cancer in combination with hormone therapies [5]. Here, CDK4 is linked to breast cancer.