The WT1 gene is a tumor-suppressor gene that was isolated as a gene responsible for Wilms' tumor, a childhood kidney neoplasm. We have previously reported that the WT1 gene is strongly expressed in leukemia cells with an increase in its expression levels at relapse and an inverse correlation between its expression levels and prognosis, thus making it a novel tumor marker for leukemic blast cells. Furthermore, WT1 antisense oligomers have been found to inhibit the growth of leukemic cells. These results strongly suggested the involvement of the WT1 gene in human leukemogenesis. The present study was performed to prove our hypothesis that the WT1 gene plays a key role in leukemogenesis and performs an oncogenic function in hematopoietic progenitor cells, rather than a tumor-suppressor gene function. 32D cl3, an interleukin-3-dependent myeloid progenitor cell line, differentiates into mature neutrophils in response to granulocyte colony-stimulating factor (G-CSF). However, when transfected wild-type WT1 gene was constitutively expressed in 32D cl3, the cells stopped differentiating and continued to proliferate in response to G-CSF. As for signal transduction mediated by G-CSF receptor (G-CSFR), Stat3alpha was constitutively activated in wild-type WT1-infected 32D cl3 in response to G-CSF, whereas, in WT1-uninfected 32D cl3, activation of Stat3alpha was only transient. However, most interesting was the fact that G-CSF stimulation resulted in constitutive activation of Stat3beta only in wild-type WT1-infected 32D cl3, but not in WT1-uninfected 32D cl3. Thus, WT1 expression constitutively activated both Stat3alpha and Stat3beta. A transient activation of Stat1 was detected in both wild-type WT1-infected and uninfected 32D cl3 after G-CSF stimulation, but no difference in its activation was found. No activation of MAP kinase was detected in both wild-type WT1-infected and uninfected 32D cl3 after G-CSF stimulation. These results demonstrated that WT1 expression competed with the differentiation-inducing signal mediated by G-CSFR and constitutively activated Stat3, resulting in the blocking of differentiation and subsequent proliferation. Therefore, the data presented here support our hypothesis that the WT1 gene plays an essential role in leukemogenesis and performs an oncogenic function in hematopoietic progenitor cells and represent the first demonstration of an important role of the WT1 gene in signal transduction in hematopoietic progenitor cells.