(A) Cell lysates were evaluated for phospho-pRb and pRb expression by Western blot. β-actin is shown as loading control. (B) MB231 and MB453 cells, and (C) MB468 and CAL148 were treated with palbociclib at the indicated concentrations and cell proliferation was assessed by MTT assay after 24, 72, and 120 h incubation. Results of MTT assay are expressed as the ratio of control at the time, and are mean values ± SD at each palbocilcib concentration, from triplicate measurements for each treatment.