Palbociclib 2HCL (PD-0332991 dihydrochloride), the dihydrochloride salt of Palbociclib, is a highly specific inhibitor of Cdk4 and Cdk6, with IC50 values of 11 nM and 16 nM, respectively. It is a potent antiproliferative agent against retinoblastoma (Rb)-positive tumor cells in vitro, inducing a selective G1 cell cycle arrest and reducing phosphorylation at Ser780 and Ser795 on the Rb protein.