As a highly selective CDK inhibitor, PD 0332991 can potently inhibit CDK4 and CDK6 with IC50 of 11 nM and 16 nM, but shows no activity against other kinases. PD0332991 was able to prevent upregulation of cyclin D1 and CDK6 as well as CDK4 and phosphorylated pRb and this correlated with a more profound reversal of hyperplasia and dysplasia.