Palbociclib (trade name: Ibrancea, approved by FDA in February 2015 to treat HR+/HER2- breast cancer), amebaciclib and ribociclib (trade name: Kisqali, approved by FDA in March 2017 to treat HR+/HER2- breast cancer) are recently developed CDK4/6 inhibitors currently undergoing clinical testing as potential chemotherapeutics for the treatment of primary or secondary brain tumors [29]. Here, CDK4 is linked to breast cancer.