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MicroRNA142 (MIR142) is a target of chromosome translocations and mutations in human B-cell lymphomas. We analyzed an aggressive B-cell lymphoma carrying t(8;17)(q24;q22) and t(6;14)(p21;q32), and sought to explore the role(s) of MIR142 in lymphomagenesis. t(8;17)(q24;q22) involved MYC on 8q24 and pri-MIR142 on 17q22. MYC was activated by a promoter substitution by t(8;17)(q24;q22). t(8;17)(q24;q22) was an additional event after t(6;14) (p21;q32), which caused the over-expression of CCND3. Southern blot analyses revealed that the MIR142 locus was deleted from the affected allele, whereas Northern analyses showed over-expression of MIR142 in tumor cells. Although previous studies reported an over-expression of mutations in MIR142 in B-cell lymphomas, limited information is available on the functions of MIR142 in lymphomagenesis. Therefore, we generated bone marrow transplantation (BMT) and transgenic (Eμ/mir142) mice, which showed enforced expression in hematopoietic progenitor cells and B cells, respectively. BMT mice showed decreased numbers of all lineage-positive cells, particularly B cells, in peripheral blood. Eμ/mir142 mice showed decreased numbers of IgM-positive splenocytes, and exhibited altered B-cell phenotypic changes induced by lipopolysaccharide. Our results suggest that over-expression of MIR142 alters B-cell differentiation, implying multi-step lymphomagenesis together with MYC activation and CCND3 over-expression. [ABSTRACT FROM AUTHOR]