RB1 and cancer: The correlations between protein levels of Rb1, cyclin E1, cyclin E1/Rb ratio, and palbociclib IC50 were also tested in a large dataset of cancer cell lines analyzed by reverse-phase protein arrays.21 Rb, cyclin E1, and the cyclin E1/Rb ratio correlated with palbociclib IC50, with the cyclin E1/Rb ratio performing better than cyclin E1 and Rb alone in predicting palbociclib resistance, both in the pan-cancer and the BC datasets (Fig. 5c).