miRNAs are small RNAs that guide the RNA-induced silencing complex to mRNA targets, destabilizing them and inhibiting their translation. Though much has been learned about their involvement in organism development and function, some striking puzzles remain. On the one hand it has been shown that miRNAs play essential roles in development, and the preferential targeting of transcription factors (TFs) by miRNAs suggests that they ”coordinate” to regulate gene expression. Indeed, studies in the past year came to the striking conclusion that, on their own or in combination with TFs, miRNAs can induce reprogramming of somatic cells into induced pluripotent stem cells (iPSC). On the other hand, high-throughput experimental studies suggest that miRNAs have largely a ”fine-tuning” function; even miRNAs that promote cell reprogramming induce only mild changes in the expression levels of their targets in transfection experiments. These small effects on individual genes must, however, confer a substantial selective advantage, because many target sites remain conserved over long evolutionary distances. In this project I first propose to investigate a hypothesis that may explain this apparent contradiction, namely that miRNAs reduce the cell-to-cell variation in gene expression, rather than the average levels of their targets. I will then use miRNA-mediated reprogramming of somatic cells into iPSCs as a model system to directly investigate the interplay and ”coordination” between miRNAs and TFs in determining cell identity and differentiation. More specifically, I propose to study the sequence of regulatory events that leads from the induction of a few miRNAs, through the perturbation of TF activities, to the activation of ”stemness” genes. Finally, following up on preliminary results obtained in my lab, I will investigate the function of miRNA targeting in the nucleus, that potentially couples transcriptional and post-transcriptional regulation more directly.