CDK4 and breast cancer: Moreover, knocking-down of βTrCP1 suppressed HFF1 cell proliferation (Supplementary Fig. 3k), in which RB1 proteins were mainly in hypophosphorylated form and supposed to have higher affinity with βTrCP1 (Supplementary Fig. 3l), and enhanced the inhibitory effect of CDK4/6i on MCF7 breast cancer cell proliferation (Supplementary Fig. 3m).