"miRNAs are small RNAs that guide the RNA-induced silencing complex tomRNA targets, destabilizing them and inhibiting theirtranslation. Much has been learned about their involvement in organismdevelopment and function, yet some striking puzzles remain. On the onehand it has been shown that miRNAs are essential for development, andthe preferential targeting of transcription factors (TFs) by miRNAssuggests that miRNAs and TFs ""coordinate"" to regulate geneexpression. Furthermore, studies in the past year concluded that, ontheir own or in combination with TFs, miRNAs can induce reprogrammingof somatic cells into induced pluripotent stem cells (iPSC). On theother hand, high-throughput measurements of mRNA and protein levelchanges upon miRNA transfections suggest that miRNAs have largely a""fine-tuning"" function. These small effects on individual genes must,however, confer a substantial selective advantage, because many targetsites remain conserved over long evolutionary distances. Here I firstpropose to investigate the hypothesis that instead of primarilyaffecting the average levels of target genes, miRNAs reduce thecell-to-cell variation in gene expression, affectingprecisely the steps that determine the intrinsic noise. I will thenuse miRNA-mediated reprogramming of somatic cells into iPSCs as amodel system to directly investigate the ""coordination"" between miRNAsand TFs in determining cell identity and differentiation. Throughdetermination of miRNA targets with Argonaute crosslinking andimmunoprecipitation, mRNA sequencing and methylated DNAimmunoprecipitation I attempt to retrace the regulatory interactionsthat lead from induction of a few miRNAs, through perturbation of TFactivities, to the activation of ""stemness"" genes. Finally, followingup on preliminary results obtained in my lab, I will investigate thefunction of miRNA targeting in the nucleus, that potentially couplestranscriptional and post-transcriptional regulation more directly."