ODC activity displays two peaks during the entire cell cycle process, one associated with the G1/S transition and the other associated with the S/G2 and G2/M phases,39 suggesting that polyamines also play key roles in cell cycle progression. A number of studies reported that polyamine depletion arrests cells at the G1 phase.40–42 Other effects, including arrest at the S or G2/M phase, have been reported in different cell lines,43–45 suggesting that polyamine depletion affects the cell cycle in a cell-type specific manner. Our data showed that knocking down ODC expression arrested ESCC cells at the G2/M phase. Correspondingly, the percentage of cells in the S phase was decreased, with no significant difference at the G1 phase (Fig. 3b). Cell cycle progression is mediated by the family of cyclin-dependent kinases, which comprise a catalytic subunit and requisite positive regulatory subunits known as cyclins. Generally, positive regulation of CDK activity is mediated by the accumulation of cyclins, whereas negative regulation is achieved by phosphorylation of the catalytic subunit or by binding with CDK inhibitors, including p21 and p27.46 Cyclin B1 contributes to the transition of cells from the G2 to M phase and is inactivated by phosphorylation of CDK1.47 In ESCC, cyclin B1 was reported to be an independent prognostic factor in patients,48 indicating that cyclin B1 plays a key role in ESCC cell proliferation. Our results showed that polyamine depletion increased the expression of wildtype p53, which in turn directly down-regulated the transcription of cyclin B1 mRNA and inactivated the CDK1/cyclin B1 complex by phosphorylation of CDK1. Moreover, as a transcription target of p53, p21 also can effectively inhibit the kinase activity of CDK1.49 Altogether, our data suggested that polyamine depletion arrested ESCC cells in the G2/M phase by decreasing the activity of the CDK1/cyclin B1 complex.