Down-regulation of miRNA-21 inhibited A549 cell proliferation and cycle progress after IR. A, A549 cells were transfected with miRNA-21 RNAi and scrambled RNAi vector. Expression of miRNA-21 was detected in 48 hours after transfection. miRNA-21 level decreased in RNAi cells. B, Statistical plots of 3 independent cell growth assay.A549 cells were irradiated with 6 Gy γ-ray in 48 h after transfection. Then cells were harvest at 1, 2, 3, 4, 5 and 6 day after irradiation, respectively and cell growth was monitored. The proliferation ability of miRNA-21 knock down cells was lower than that of control groups. C, Cells were exposed to 6 Gy γ-ray in 48 hours after transfection. Then these cells were harvested and cell cycle was examined by flow cytometry. Percentage of cells in different cell cycle phases was plotted and results were represented as mean ± s.d. Comparing with the control groups, the percentage of cells at G1 phase was significantly increased, whereas the population of cells at S phase significantly decreased in miRNA-21 knock down group (p<0.01).