In a phase-1 clinical trial (NCT00420056) by Leonard et al., CDK4/6 inhibitor palbociclib (PD0332991) was given to MCL patients (n = 17; 71% were at high/intermediate risk according to MCL International Prognostic Index score) that induced early G1 cells arrest and tumor regression in some patients. The gene discussed is CDK4; the disease is mantle cell lymphoma.