microRNAs are small naturally occurring non-coding RNAs that are 20 to 25 nucleotides in length. microRNAs regulate expression of target genes through sequence-specific hybridization to the 3′ untranslated region (UTR) of messenger RNAs and either block translation or direct degradation of their target messenger RNAs. This novel class of small non-coding RNAs was first discovered in the model organism C. elegans in 1993 and since then microRNAs have been identified in almost every species, including humans. They are highly conserved and research has shown that microRNAs function as important regulators of gene expression and play a role in multiple cellular processes.
microRNAs play an integral role in numerous biological processes, including the immune response, cell-cycle control, metabolism, viral replication, stem cell differentiation and human development. Most microRNAs are conserved across multiple species, indicating the evolutionary importance of these molecules as modulators of critical biological pathways. Indeed, microRNA expression or function is significantly altered in numerous diseases including cancer and fibrosis, as well as CNS, metabolic and inflammatory disorders. Targeting pathways of human disease with microRNA-based drugs represents a novel and potentially powerful therapeutic approach. There are about 2,600 predicted mature human microRNAs, 600 of which are considered to be well validated and provide a large space of novel target opportunities.