MicroRNAs (miRNAs) are small RNA molecules that regulate gene expression 
post-transcriptionally. After the discovery of the first miRNA in the roundworm 
Caenorhabditis elegans, these short regulatory RNAs have been found to be an 
abundant class of RNAs in plants, animals, and DNA viruses. About 3% of human 
genes encode for miRNAs, and up to 30% of human protein coding genes may be 
regulated by miRNAs. MicroRNAs play a key role in diverse biological processes, 
including development, cell proliferation, differentiation, and apoptosis. 
Accordingly, altered miRNA expression is likely to contribute to human disease, 
including cancer. This review will summarize the emerging knowledge of the 
connections between human miRNA biology and different aspects of carcinogenesis. 
Various techniques available to investigate miRNAs will also be discussed.