The WT1 targeted siRNA transfection induced the cell cycle arrested at the G1 phase through induction of p21 expression in WT1+ A549 cell lines but has little effect in WT1-PC14 lung cancer cell line; although we can’t detected the apoptotic cells by flow cytometry 24 h post WT1 siRNA transfection alone, which could due to the targeted siRNA we designed was targeting the Exon 8; we actually found that in A549 cells, the WT1 inhibition increased the expssion of proapoptotic protein BAX, BAK, and decreased the antiapoptotic protein Bcl-XL, Mcl-1 expression. This evidence concerns the gene MCL1 and lung cancer.