Activation of transcription factor STAT3 is involved in cell proliferation,differentiation,and cell survival.Constitutive activation of STAT3 pathway has been associated with the oncogenesis of various types of cancers.It has been reported that STAT3 plays a key role in the G1 to S phase cell cycle transition induced by the cytokine receptor subunit gp130,through the upregulation of cyclins D1,D2,D3,A,and Cdc25A and the concomitant downregulation of p21 and p27.However,its role in mediating G2-M phase transition has not been studied.The cyclin B1/Cdc2 complex is widely accepted as the trigger of mitosis in all organisms and is believed to be necessary for progression through S phase and keep active during the G2-M transition and progression.In the present study,we found that activation of STAT3 stimulates cyclin B1 and Cdc2 expressions.Deletion and site-directed mutations on cyclin B1 and Cdc2 promoters indicated that E2F element mediates the upregulation of these two promoters in a STAT3-dependent manner.The findings reported here demonstrated that STAT3 participates in modulating G2-M phase checkpoint by regulating gene expressions of cyclin B1 and Cdc2 via E2F.