Objective To explore the role of p21 in ionizing radiation-induced changes in protein levels during the G2/M transition and long-term G2 arrest.Methods Protein expression levels were assessed by western blot in the human uveal melanoma 92-1 cells after treatment with ionizing radiation.Depletion of p21 was carried out by employing the siR NA technique.Cell cycle distribution was determined by flow cytometry combined with histone H3 phosphorylation at Ser28,an M-phase marker.Senescence was assessed by senescenceassociated-β-galactosidase(SA-β-gal) staining combined with Ki67 staining,a cell proliferation marker.Results Accompanying increased p21,the protein levels of G2/M transition genes declined significantly in 92-1 cells irradiated with 5 Gy of X-rays.Furthermore,these irradiated cells were blocked at the G2 phase followed by cellular senescence.Depletion of p21 rescued radiation-induced G2 arrest as demonstrated by the upregulation of G2/M transition kinases,as well as the high expression of histone H3 phosphorylated at Ser28.Knockdown of p21 resulted in entry into mitosis of irradiated 92-1 cells.However,cells with serious DNA damage failed to undergo cytokinesis,leading to the accumulation of multinucleated cells.Conclusion Our results indicated that p21 was responsible for the downregulation of G2/M transition regulatory proteins and the bypass of mitosis induced by irradiation.Downregulation of p21 by siR NA resulted in G2-arrested cells entering into mitosis with serious DNA damage.This is the first report on elucidating the role of p21 in the bypass of mitosis. Objective To explore the role of p21 in ionizing radiation-induced changes in protein levels during the G2 / M transition and long-term G2 arrest. Methods Protein expression levels were assessed by western blot in the human uveal melanoma 92-1 cells after treatment with ionizing radiation. Depiption of p21 was carried out by employing the siR NA technique. Cell cycle distribution was determined by flow cytometry combined with histone H3 phosphorylation at Ser28, an M-phase marker. Senescence was assessed by senescenceassociated-β-galactosidase (SA- β-gal) staining combined with Ki67 staining, a cell proliferation marker. Results Accompanying increased p21, the protein levels of G2 / M transition genes declined significantly in 92-1 cells irradiated with 5 Gy of X-rays. were blocked at the G2 phase followed by cellular senescence. Depletion of p21 rescued radiation-induced G2 arrest as demonstrated by the upregulation of G2 / M transition kinases, as well as the high e Expression of histone H3 phosphorylated at Ser28. Knockdown of p21 resulted in entry into mitosis of irradiated 92-1 cells. However, cells with serious DNA damage failed to under cytokinesis, leading to the accumulation of multinucleated cells. Confluence Our results indicated that p21 was responsible for the downregulation of G2 / M transition regulatory proteins and the bypass of mitosis induced by irradiation. Downregulation of p21 by siR NA resulted in G2-arrested cells entering into mitosis with serious DNA damage. this is the first report on elucidating the role of p21 in the bypass of mitosis.