microRNAs （miRNAs） constitute a unique class of endogenous small non-coding RNAs that regulate gene expression post-transcriptionally. Studies over the past decade have uncovered a r^curring paradigm in which miRNAs are key regulators of cellular behavior under various physiological and pathological conditions. Most surprising is the recent observation that miRNAs have emerged as competent players in somatic cell reprogramming, suggesting an especially significant role for these small RNAs in cell fate settings. Here, we discuss the possible mechanisms underlying miRNA-mediated cell programming （i.e., the development and differentiation of embryonic stem cells） and reprogramming （i.e., turning somatic cells into pluripo- tent stem cells or other lineages）, and provide a ＂Helm＂ model of miRNAs in cell fate decision and conversion.
