TheMLLgene, at 11q23, undergoes chromosomal translocation with a large number of partner genes in both acute lymphoblastic and acute myeloid leukemia (AML). We report a novel t(9;11;19)(p22;q23;p13.3) disruptingMLLin an infant AML patient. The 5′ end ofMLLfused to chromosome 9 sequences on the der(11), whereas the 3′ end was translocated to chromosome 19. We developed long-distance inverse–polymerase chain reaction assays to investigate the localization of the breakpoints on der(11) and der(19). We found that intron 5 ofMLLwas fused to intron 5 ofMLLT3at the der(11) genomic breakpoint, resulting in a novel in-frameMLLexon 5–MLLT3exon 6 fusion transcript. On the der(19), a novel gene annotated asFLJ10374was disrupted by the breakpoint. Using reverse transcription–polymerase chain reaction analysis, we showed thatFLJ10374is ubiquitously expressed in human cells. Transfection of the FLJ10374 protein in different cell lines revealed that it localized exclusively to the nucleus. In serum-starved NIH-3T3 cells, the expression ofFLJ10374decreased the rate of the G1-to-S transition of the cell cycle, whereas the suppression ofFLJ10374through short interfering RNA increased cell proliferation. These results indicate that FLJ10374 negatively regulates cell cycle progression and proliferation. Thus, a single chromosomal rearrangement resulting in formation of theMLL–MLLT3fusion gene and haplo-insufficiency ofFLJ10374may have cooperated to promote leukemogenesis in AML with t(9;11;19). © 2006 Wiley-Liss, Inc.