Effects of PD 0332991 on phosphorylation of retinoblastoma gene product. PD 0332991 blocks phosphorylation of retinoblastoma gene product pRb in sensitive cell lines but not in resistant cell lines. (a) In neither group does total pRb change significantly with treatment. (b) PD 0332991 significantly blocks phoshorylation of pRb (phospho-Rb) at serine 780 in sensitive cell lines (IC50 < 150 nM), but not in resistant cell lines (IC50 > 1,000 nM). All cell lines were treated with 100 nM PD 0332991 for the times specified and western blots were performed as described in Materials and methods.