CDK4 and breast cancer: In striking contrast, Palbociclib (PD0332991), a highly specific inhibitor CDK4 (IC50 = 11 nM) and CDK6 (IC50 = 16 nM), is well tolerated and has shown to efficiently suppress growth of estrogen receptor-positive breast cancer [163,164], non-small cell lung carcinoma [154], and various hematologic malignancies including T cell acute lymphoblastic leukemia [164,165], Burkitt lymphoma [156], and mantle cell lymphoma [166].