The eukaryotic complexity involves the expression and regulation of genes via 
RNA-DNA, RNA-RNA, DNA-protein and RNA-protein interactions. Recently, the role 
of RNA molecules in the regulation of genes in higher organisms has become more 
evident, especially with the discovery that about 97% of the transcriptional 
output in higher organisms are represented as noncoding RNAs: rRNA, snoRNAs, 
tRNA, transposable elements, 5' and 3' untranslated regions, introns, intergenic 
regions and microRNAs. MicroRNAs function by negatively regulating gene 
expression via degradation or translational inhibition of their target mRNAs and 
thus participate in a wide variety of physiological and pathological cellular 
processes including: development, cell proliferation, differentiation, and 
apoptosis pathways. MicroRNA expression profiles in many types of cancers have 
been identified. Recent reports have revealed that the expression profiles of 
microRNAs change in various human cancers and appear to function as oncogenes or 
tumor suppressors. Abnormal microRNA expression has increasingly become a common 
feature of human cancers. In this review, we summarize the latest progress on 
the involvement of microRNAs in different types of cancer and their potential 
use as potential diagnostic and prognostic tumor biomarkers in the future.