To investigate the expressions of TET2 mRNA in bone marrow CD3(+) and CD34(+) cells of the patients with myelodysplastic syndromes (MDS) and to study the effect of silencing TET2 by small interfering RNA (siRNA) on the biological characteristics of CD34(+) cells.CD3(+) and CD34(+) cells were sorted by magnetic activated cell-sorting system from bone marrow of MDS patients and controls. The mRNA expressions of TET2 in bone marrow CD3(+) and CD34(+) cells of 28 MDS patients and 20 controls were detected by qPCR. The silencing effect of RNA interference (RNAi) on TET2 expression in CD34(+) bone marrow cells of normal control was identified by qPCR and Western blot analysis. The cell cycle kinetics and cell apoptosis were then detected by flow cytometry.The expression of TET2 mRNA in CD3(+) and CD34(+) cells was down-regulated in MDS compared with that in controls [(0.16 ± 0.11) vs. (1.05 ± 0.32) (P<0.001); (0.58 ± 0.26) vs. (1.25 ± 0.94) (P<0.005)]. The siRNA targeting TET2 suppressed the expression of TET2 in normal CD34(+) cells. Meanwhile, the proliferation activity was significantly enhanced [G0/G1: (87.82 ± 8.25)% vs. (92.65 ±7.06)% and (93.60 ± 5.54)%, P<0.05; S: (11.50 ± 8.31)% vs. (6.92 ± 7.04)% and (5.95 ± 5.53)%, P<0.05] and the apoptosis rate was declined [(21.28 ± 9.73)% vs. (26.17 ± 9.88)% and (26.20 ± 9.78)%] in the cells which transfected with TET2 siRNA as compared to those in the cells transfected with scrambled siRNA and control cells.The TET2 expression of in CD3(+) and CD34(+) cells of MDS patients was decreased. Suppression of TET2 expression renders the CD34(+) cells harboring more aggressive phenotype. This preliminary finding suggests that CD34(+) cells lowering expression of TET2 may play an oncogenic role on myeloid tumor and CD3(+) T cells of MDS patients may be derived from the malignant clone.