Stably knocking down CDK4 expression suppresses cell proliferation and cell cycle progression in vitro in NPC. A. CDK4 protein expression was stably suppressed by lentivirus-mediated shRNA in NPC cells. B. In vitro viability of NPC cells was decreased in CDK4-suppressed cells compared to PLV-Ctr cells by MTT assay. C. In vitro proliferative ability of NPC cells was significantly decreased in CDK4-suppressed cells compared to PLV-Ctr cells by colony formation assay. D. Stably downregulated CDK4 expression blocked cell cycle transition from G1 to S in shRNA-CDK4-2 and 3 cells. Data are presented as mean ± SD for three independent experiments (*p < 0.05).