AIM: To explore the effect of microRNA-221( miR-221) on the proliferation of lung cancer A549 cells,and to investigate its mechanism. METHODS: The A549 cells were transfected with miR-221 mimics by Lipofectamine 2000. The expression of miR-221 was detected by RT-q PCR. The expression of PTEN at mRNA and protein levels was detected by RT-q PCR and Western blot,respectively. The cell proliferation was examined by CCK-8 assay and colony formation assay. The 3'-UTR of PTEN was cloned into luciferase reporter ve...