LAUSR

During the last decade, four types of silencingrelated small RNAs have been discovered: small interfering RNAs (siRNAs), microRNAs (miRNAs), repeat associated small interfering RNAs (rasiRNAs) and piwi-interacting RNAs (piRNAs). MicroRNAs are known as a class of endogenous, non-coding small RNA molecules (18-25 nucleotides) that are able to post-transcriptionally regulate gene expression through sequence-specific base pairing to mRNA. The first discovered miRNA, Lin-4, known to control the timing of C. elegans, was described in 1993 by Lee and colleagues. Another gene in the C. elegans heterolarval development, named let-7, was discovered 7 years after Lin-4 (34). Most mammalian miRNA genes (about 80%) have been identified in the intron region of either protein coding or non-protein coding transcripts. Only a small proportion of miRNA genes (20%) are located in the exon region of non-coding RNAs. Interestingly, some miRNA genes could be related to either exonic or intronic miRNAs, depending on the alternative splicing pattern of the host genes (31), (47). These small molecules play a crucial role in posttranscriptional gene expression control (47) and are involved in many gene regulation processes (13). A number of previous studies have shown that eukaryotes use miRNAs to regulate functions related to developmental timing, cell proliferation, differentiation, signaling, the programming of death (apoptosis) pathways (27), fat metabolism in flies in particular cell types (8), neuronal patterning in nematodes (30), and MicroRNA function in domestic animal physiology and diseases: a promising diagnostic tool for veterinary use*)