CDK4 and breast neoplasm: Preclinical studies using breast tumor cell lines (Finn et al, 2009; Dean et al, 2010; Miller et al, 2011), mouse models (Choi et al, 2012), and ex vivo patient tumor cells (Dean et al, 2012) have demonstrated the efficacy of inhibiting CDK4/6 by PD0332991 (palbociclib) to arrest proliferation.