MicroRNA-155 (miR-155) is an oncogenic miRNA upregulated in various tumor types and leukemias and has been suggested as a potential drug target. Based on our previous work detecting high miR-155 levels in response to Meis1 overexpression in a murine Hox leukemia model, we show here the relationship among HOXA9 , MEIS1 , and miR-155 levels in MLL -translocated acute myeloid leukemia (AML) patients. Using mouse bone marrow cells transformed by MLL -fusion genes expressing graduated levels of Meis1 , we show a positive correlation between miR-155 and Meis1 . However, using a miR-155-knockout mouse model, we show that the absence and the depletion of miR-155 have no effect on leukemia formation or progression. We also show for the first time that miR-155 levels are correlated with MLL translocations, but that miR-155 expression is dispensable for the formation of AML and has no effect on leukemia progression. [ABSTRACT FROM AUTHOR]