To investigate the effect of a novel p21-modulating protein WISp39 on proliferation, apoptosis and cell cycle of leukemia cells, the plasmid pLenti6/V5-WISp39 was constructed and transfected into the human myelocytic leukemia cell line-U937 cells. The expression of WISp39 was detected by real-time PCR at 48 hours after transfection, proliferation of U937 cells assayed by CCK-8, apoptosis and cell cycle were determined by flow cytometry. The results showed that plasmid pLenti6/V5-WISp39 could readily enhance the expression of WISp39 in U937 cells. A significant growth inhibition (37.6%) was observed in cells tranfected with pLenti6/V5-WISp39, while the control plasmid pLenti6/V5-lacZ showed little effect on U937 growth. Further analysis revealed that pLenti6/V5-WISp39 did not show obvious apoptosis induction effect, but it could really regulate U937 proliferation via cell cycle modulation. Compared with pLenti6/V5-lacZ, pLenti6/V5-WISp39 resulted in increase of cells in G(0)/G(1) phase by 10% at 48 hours after transfection. It is concluded that the WISp39 gene has no significant apoptosis induction effect on leukemic cells, but it can increase cells at G(0)/G(1) phase via effect on cell cycle, thus inhibiting the U937 proliferation. This result means WISp39 gene can act as a negative modulator on tumour cells.