MicroRNAs (miRNAs) are small non-coding RNA molecules that are widely involved 
in cancer-related processes. The microRNA-21 (miR-21) has been identified as the 
only miRNA overexpressed in a variety of cancers, including leukemia. However, 
the function of miR-21 is yet unknown in chronic myelogenous leukemia (CML). 
Antisense oligonucleotides (ASOs), as inhibitors of miRNAs, have already been 
applied to therapeutic development and functional identification in miRNA 
research. In this study, we found that the antisense inhibition of miR-21 in 
K562 cells suppressed cell migration, promoted cell apoptosis, and inhibited 
cell growth, and up-regulated the expression of the tumor suppressor gene PDCD4. 
Meanwhile, pre-miRNA-21 increased migration and decreased cell apoptosis without 
affecting proliferation. We also validated that PDCD4 is a functional target of 
miR-21 in K562 cells. These effects of miR-21 might be partially due to its 
regulation of PDCD4. Our data suggest that miR-21 may play an oncogenic role in 
the cellular processes of CML, and antisense inhibition of miR-21 may therefore 
be useful as CML therapy.