MicroRNAs are key regulators of various fundamental biological processes and, 
although representing only a small portion of the genome, they regulate a much 
larger population of target genes. Mature microRNAs (miRNAs) are single-stranded 
RNA molecules of 20-23 nucleotide (nt) length that control gene expression in 
many cellular processes. These molecules typically reduce the stability of 
mRNAs, including those of genes that mediate processes in tumorigenesis, such as 
inflammation, cell cycle regulation, stress response, differentiation, apoptosis 
and invasion. MicroRNA targeting is mostly achieved through specific 
base-pairing interactions between the 5' end ('seed' region) of the miRNA and 
sites within coding and untranslated regions (UTRs) of mRNAs; target sites in 
the 3' UTR diminish mRNA stability. Since miRNAs frequently target hundreds of 
mRNAs, miRNA regulatory pathways are complex. Calin and Croce were the first to 
demonstrate a connection between microRNAs and increased risk of developing 
cancer, and meanwhile the role of microRNAs in carcinogenesis has definitively 
been evidenced. It needs to be considered that the complex mechanism of gene 
regulation by microRNAs is profoundly influenced by variation in gene sequence 
(polymorphisms) of the target sites. Thus, individual variability could cause 
patients to present differential risks regarding several diseases. Aiming to 
provide a critical overview of miRNA dysregulation in cancer, this article 
reviews the growing number of studies that have shown the importance of these 
small molecules and how these microRNAs can affect or be affected by genetic and 
epigenetic mechanisms.