MicroRNAs are non-coding small RNAs of ~22 nucleotides that regulate the gene expression by base-paring with target mRNAs, leading to mRNA cleavage or translational repression. It is currently estimated that microRNAs account for ~ 1% of predicted genes in higher eukaryotic genomes and that up to 30% of genes might be regulated by microRNAs. However, only very few microRNAs have been functionally characterized and the general functions of microRNAs are not globally studied.

We systematically analyzed the expression patterns of microRNA targets using several public microarray profiles and found that the expression levels of microRNA targets are significantly lower in all mouse and Drosophila tissues than in the embryos and that microRNA targets are dramatically excluded from the tissue-specifically expressed gene groups.

These results strongly suggest that the global functions of microRNAs are largely involved in driving tissue differentiation and maintaining tissue identity rather than in tissue-specific physiological functions. In addition, these findings imply that disruption of microRNA functions might cause delineation of differentiated cells, a crucial step towards carcinogenesis.