Although there have been many efforts to develop selective inhibitors of CDK4/CDK6, initial studies with palbociclib, an FDA-approved selective small molecule inhibitor of CDK4 and CDK6 [21, 22], revealed that the drug was cytostatic when used as a monotherapy for MCL, resulting in disease stabilization with an 18% overall response rate [23]. Here, CDK4 is linked to mantle cell lymphoma.