MicroRNAs (miRNAs) are small noncoding RNA molecules that negatively control 
expression of target genes in animals and plants. The microRNA-21 gene (mir-21) 
has been identified as the only miRNA commonly overexpressed in solid tumors of 
the lung, breast, stomach, prostate, colon, brain, head and neck, esophagus and 
pancreas. We initiated a screen to identify miR-21 target genes using a reporter 
assay and identified a potential miR-21 target in the 3'-UTR of the programmed 
cell death 4 (PDCD4) gene. We cloned the full-length 3'-UTR of human PDCD4 
downstream of a reporter and found that mir-21 downregulated, whereas a modified 
antisense RNA to miR-21 upregulated reporter activity. Moreover, deletion of the 
putative miR-21-binding site (miRNA regulatory element, MRE) from the 3'-UTR of 
PDCD4, or mutations in the MRE abolished the ability of miR-21 to inhibit 
reporter activity, indicating that this MRE is a critical regulatory region. 
Western blotting showed that Pdcd4 protein levels were reduced by miR-21 in 
human and mouse cells, whereas quantitative real-time PCR revealed little 
difference at the mRNA level, suggesting translational regulation. Finally, 
overexpression of mir-21 in MCF-7 human breast cancer cells and mouse epidermal 
JB6 cells promoted soft agar colony formation by downregulating Pdcd4 protein 
levels. The demonstration that miR-21 promotes cell transformation supports the 
concept that mir-21 functions as an oncogene by a mechanism that involves 
translational repression of the tumor suppressor Pdcd4.