8 MCL patient samples expressed only the cyclin D1a protein despite expressing detectable cyclin D1b mRNA….Novel small molecule CDK4/CDK6 inhibitor PD0332991 profoundly suppressed—at low nanomolar concentrations—Rb phosphorylation, proliferation, and cell cycle progression at the G0/G1 phase of MCL cells. These findings provide evidence that MCL should be very sensitive to targeted therapy aimed at functional inhibition of the cyclin D1/CDK4 complex....