MicroRNA: a new player in stem cells.

microRNAs (miRNAs) are an abundant class of endogenous non-protein-coding small RNAs, which negatively regulate gene expression at the posttranscriptional level in many developmental and metabolic processes. miRNAs regulate a variety of biological processes, including developmental timing, signal transduction, tissue differentiation and maintenance, disease, and carcinogenesis. Emerging evidence demonstrates that miRNAs also play an essential role in stem cell self-renewal and differentiation. Some miRNAs are specifically expressed in stem cells, control stem cell self-renewal, and differentiation through negatively regulating the expression of certain key genes in stem cells. Copyright 2006 Wiley-Liss, Inc.