microRNAs: a newly described class of encoded molecules that play a role in health and disease.

Micro RNAs are evolutionarily conserved, single stranded molecules of about 22 nucleotides in length and function post-transcriptionally by partial binding (partial complementarity) to the mRNA of genes. Binding of a specific miRNA to its target on an mRNA can inhibit its expression by a variety of mechanisms. Although the most common mechanism is translational repression as a result of miRNA binding to the 3'UTR of an mRNA, mechanisms involving mRNA degradation and destabilization have also been described. Micro RNAs are currently considered as "master regulators" of gene expression. Since a single miRNA can bind and consequently regulate the expression of more than 100 different transcripts it has been estimated that miRNAs may be able to regulate up to 30% of the protein-coding genes in the human genome. As a result, miRNAs receive widespread attention on their potential role in complicated biological processes and multifactorial diseases. In this review we are discussing the biogenesis of miRNAs, their mode of action as well as their role in human diseases through genetic variations on their target sites.