MicroRNAs (miRNAs) exert negative effects on gene expression and influence cell 
lineage choice during hematopoiesis. C/EBPa-induced pre-B cell-to-macrophage 
transdifferentiation provides an excellent model to investigate the contribution 
of miRNAs to hematopoietic cell identity, especially because the two cell types 
involved fall into separate lymphoid and myeloid branches. In this process, 
efficient repression of the B cell-specific program is essential to ensure 
transdifferentation and macrophage function. miRNA profiling revealed that 
upregulation of miRNAs is highly predominant compared with downregulation and 
that C/EBPa directly regulates several upregulated miRNAs. We also determined 
that miRNA 34a (miR-34a) and miR-223 sharply accelerate C/EBPa-mediated 
transdifferentiation, whereas their depletion delays this process. These two 
miRNAs affect the transdifferentiation efficiency and activity of macrophages, 
including their lipopolysaccharide (LPS)-dependent inflammatory response. 
miR-34a and miR-223 directly target and downregulate the lymphoid transcription 
factor Lef1, whose ectopic expression delays transdifferentiation to an extent 
similar to that seen with miR-34a and miR-223 depletion. In addition, ectopic 
introduction of Lef1 in macrophages causes upregulation of B cell markers, 
including CD19, Pax5, and Ikzf3. Our report demonstrates the importance of these 
miRNAs in ensuring the erasure of key B cell transcription factors, such as 
Lef1, and reinforces the notion of their essential role in fine-tuning the 
control required for establishing cell identity.