**2. Importance of microRNAs**

MiRNAs are non-codifying RNA molecules, of 18–24 nucleotides which regulate the expression of genes in a post-transcriptional level. They are found in a wide array of organisms, such as animals, plants and viruses, and in each type of cells [8, 9]. It is estimated that the genome of vertebrates codifies over 1000 different miRNAs, which regulate the expression of at least 30% of genes. The low necessary astringency for a functional interaction between miRNA/ RNAm gives the capacity to miRNAs to regulate several messengers, besides region 3′UTR of target RNAm frequently harbouring several sites of recognition of microRNAs [10]. Close to 2588 mature sequences of miRNAs have been identified in the human genome [http:// microRNA.sanger.ac.uk, version 21]. This number has rapidly increased in the last few years. Nevertheless, little is known about their specific goals and the biological functions that they undertake in the development of cancer and other illnesses [11].
