Among the key properties that distinguish adult mammalian stem cells from their more differentiated progeny is the ability of stem cells to remain in a quiescent state for prolonged periods of time 1,2 . However, the molecular pathways for the maintenance of stem cell quiescence remain elusive. Using adult muscle stem cells (" satellite cells " (SCs)) as a model system, we show that the microRNA (miRNA) pathway is essential for the maintenance of the quiescent state. SCs lacking a functional miRNA pathway spontaneously exit quiescence and enter the cell cycle. We identified quiescence-specific miRNAs in the SC lineage by microarray analysis. Among these, microRNA-489 (miR-489) is highly expressed in quiescent SCs and quickly down-regulated during SC activation. Further analysis revealed that miR-489 functions as a regulator of SC quiescence by post-transcriptionally suppressing the oncogene DEK, a protein that localizes to the more differentiated daughter cell during asymmetric division of SCs and promotes the transient proliferative expansion of myogenic progenitors. Our results provide the first evidence of the miRNA pathway in general, and a specific miRNA, miR-489, in actively maintaining the quiescent state of an adult stem cell population.