CDK4 and neoplasm: Meanwhile, it was revealed that both free PROTAC and preparations inhibited proliferation only at high PROTAC concentrations in mouse fibroblast L-929 and human umbilical vein endothelial (HUVEC) cells (Supplementary Fig. 15), due to different expression levels of CDK4/6 in tumor cells and nontumor cells (Supplementary Fig. 16).