MicroRNAs (miRNAs) are a recently discovered group of small RNA molecules 
involved in the regulation of gene expression. Analogously to mRNAs, the 
non-protein-encoding pri-miRNAs are synthesized by RNA polymerase II and 
post-transcriptionally modified by addition of a 5'-cap and a 3'-poly (A) tail. 
Subsequently, the pri-miRNA undergoes a number of processing steps in the 
nucleus and cytoplasm, and ends up as a mature approximately 22 nt miRNA, which 
can exert its function by binding to the 3'-untranslated region of a subset of 
mRNAs. Binding of the miRNA to the mRNA results in a reduced translation rate 
and/or increased degradation of the mRNA. In this way a large number of cellular 
pathways, such as cellular proliferation, differentiation, and apoptosis, are 
regulated by mi-RNAs. As corruption of these pathways is the hallmark of many 
cancers, dysregulation of miRNA biogenesis or expression levels may lead to 
tumorigenesis. The mechanisms that alter the expression of miRNAs are similar to 
those that change the expression levels of mRNAs of tumor suppressor- and 
oncogenes, i.e. gross genomic aberrations, epigenetic changes, and minor 
mutations affecting the expression level, processing, or target-interaction 
potential of the miRNA. Furthermore, expression profiling of miRNAs has been 
found to be useful for classification of different tumor types. Taken together, 
miRNAs can be classified as onco-miRs or tumor suppressor-miRs, and may turn out 
to be potential targets for cancer therapy.