To fulfill the promise of pluripotent stem cells, both embryonic and induced pluripotent stem cells, it is essential to fully understand their properties and how those properties can be manipulated to make any cell in the human body. The best way to reach that goal is to understand the relationships between these cells that grow in a culture dish in the laboratory and the equivalent cells in the developing embryo. As working with human embryos comes with many ethical concerns, an important alternative is the mouse model. Indeed, much of what we have learned in the mouse model has later been confirmed in human. Therefore, we use a combination of the mouse model and human cells to dissect the molecular basis of stem cell function and differentiation toward adult tissues. In particular, we have been focusing on a class of molecules called small RNAs that were only discovered in the 1990s and became widely appreciated in the past decade. There are several classes of these small RNAs, two of which our lab focuses on, microRNAs and endogenous siRNAs. We have found these small RNAs are essential for normal mammalian development and growth and differentiation of stem cells. In the past year, we have made significant achievements in understanding how microRNAs influence what cells become and how microRNAs themselves are regulated during early phases of cell specialization. For example, we have used microRNAs to understand how groups of genes can function together in networks to promote the de-specialization of adult cells back to embryonic stem cells. We have also used the same microRNAs to subdivide the earliest events of embryonic stem cell differentiation allowing to us follow these events during both normal development as well as during the production of induced pluripotent stem cells. Similarly, using genetic tools, we are beginning to understand the function of these microRNAs in the context of the entire organism as well as in the culture dish. Finally, we are using these microRNAs to dissect how changes in structure of DNA are regulated during early differentiation leading to the unique molecular profiles of developing cell types. Together these results are giving new and important insights into the role of small RNAs in early embryonic development. This research is expected to enable to us to more easily manipulate cell fates to produce high quality cells that could be used to study diseases of many types as well as reintroduce healthy tissue into patients with degenerative diseases.