MicroRNAs (miRNAs, miRs) are postulated to be important regulators in various cancers, including leukemia. In a large-scale miRNA expression profiling analysis of 435 human miRNAs in 52 acute myeloid leukemia (AML) samples, we found that miR-126 and its minor counterpart in biogenesis, namely, miR-126*, were specifically aberrantly overexpressed in core binding factor (CBF) AMLs including both t(8;21)/AML1-ETO and inv(16)/CBFB-MYH11 samples. Our in vitro gain- and loss-of-function experiments showed that forced expression of miR-126 inhibited apoptosis and increased the viability of AML cells, whereas the opposite effect was observed when endogenous expression of miR-126 was knocked down. In addition, through in vitro colony-forming/replating assays, we demonstrated that forced expression of miR-126 enhanced proliferation and colony-forming/replating capacity of mouse normal bone marrow progenitor cells alone and particularly, in cooperation with AML1 -ETO , a fusion gene resulting from t(8;21). Thus, our data shows that miR-126 may play a critical role in the development of CBF leukemias. In the present chapter, the materials and protocols for the study of miR-126 in leukemia are described.