Chen et al [29] have cloned about 100 unique miRNAs from mouse bone marrow. They have identified miR-223 as a mouse bone marrow specific miRNA, which is expressed mainly in myeloid lineage. miR-342 is a mouse spleen-specific miRNA which is expressed mainly in T and B cells [30]. Ectopically expression of hematopoietic miRNAs in hematopoietic stem/progenitor cells reveals that miR-181a is preferentially expressed in the B

cells in mouse bone marrow. Ectopic expression MiR-181a in hematopoietic stem/progenitor cells promotes B-cell differentiation [30]. miRNA is also involved in erythroid differentiation of CD34+ cord blood hematopoietic progenitor cells. The level of miR-221 and miR-222 were down-regulated in erythroid progenitor cells compared with CD34+ cord blood hematopoietic stem cells. Furthermore, transfection of CD34+ progenitor cells with miR-221 and miR-222 caused impaired proliferation and accelerated differentiation into erythropoietic cells, [31]. In human myeloid progenitor cells, miR-223 expression is low whereas its expression is upregulated following retinoic acid -induced differentiation. Furthermore, knockdown of miR-223 impaired the differentiation response to inducer. These studies demonstrate miR-223 plays a crucial function during granulopoiesis [32]. It seems like that different lineage of HSC exhibits specific miRNA marker, which may be used to define the progenitor cells, but more data is needed to support this hypothesis.

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