A role for microRNAs in the development of the immune system and in the pathogenesis of cancer.

MicroRNAs are a growing class of endogenous small non-coding RNAs that regulate gene expression by binding to target messenger RNAs and inducing translational repression, cleavage or destabilization of the target. Because each miRNA potentially can regulate expression of a distinct set of genes, it is conceivable that the differential expression of different miRNAs might profoundly influence the repertoire of genes that are expressed during development, differentiation or disease. Here, we provide background on the biogenesis and function of miRNAs, and discuss how miRNA-mediated regulation can influence tumorigenesis as well as normal development and function of cells of the immune system.