A t (4; 11)(q21; q23) has been described in 50–70% of cases of infant acute lymphoblastic leukemia and, less frequently, in cases of pediatric and adult acute lymphoblastic leukemia and acute myeloid leukemia (AML). In t (4; 11)(q21; q23) leukemias, the AF4 gene has been cloned as a fusion partner of the MLL gene. A human myeloid leukemia cell line, chronic neutrophilic leukemia (CNL) BC1, was established from a peripheral blood specimen of a patient with CNL in leukemic transformation. As with the original leukemia cells, the established line had at (4; 11)(q21; q23). We showed that the MLL gene on 11q23 was fused to the FLJ10849 gene on 4q21. The protein encoded by FLJ10849 belongs to the septin family, sharing highest homology with human SEPT6, which is one of the fusion partners of MLL in t (X; 11)(q13; q23) AML. Our results suggest that FLJ10849 might define a new septin family particularly involved in the pathogenesis of 11q23-associated leukemia. The established cell line, CNLBC1, could provide a useful model for analyzing the pathogenesis of MLL-septin leukemias and chronic neutrophilic leukemia.