CDK4 and breast cancer: Palbociclib (PD-0332991) has been shown to reduce carboplatin-induced myelosuppression by protecting hematopoietic progenitor cells in WT mice and was effective in decreasing thrombocytopenia an Rb-deficient mouse model of breast cancer.28 Another recent study examined the use of palbociclib in reducing radiation-induced intestinal injury in mice.29 Pre-treatment with the CDK4/6 inhibitor blocked crypt cell proliferation via G1 arrest resulting in decreased apoptosis, enhanced stem cell survival, and improved crypt regeneration.