Project Summary Reprogramming is the conversion of a somatic cell to a pluripotent stem cell. This technique is now routinely used in laboratories around the world, but the vast majority of reprogrammed stem cell lines are not developmentally fully competent, compromising their utility in research and therapy. Reprogramming induces DNA damage, which can have lasting consequences on the quality of the resulting cells. Our studies have shown that DNA damage during reprogramming is induced by abnormalities in DNA replication. However, the cause of DNA damage, the mechanisms of repair, and the developmental consequences of the damage are not well understood. The strength of this proposal is that with the experimental systems used, we are able to identify the specific type of damage induced by reprogramming, and the molecular mechanisms required for repair: we are able to distinguish the role of double strand break HR from the role of stalled replication fork stability. We are also able to distinguish the effect of genome instability on reprogramming efficiency from incomplete transcriptional transitions. We are able to map the sites in the genome with reprogramming-induced damage, and we are able to identify pathways that can be used to increase genome stability and potentially improve developmental competence of reprogrammed stem cells. These studies will provide a mechanistic understanding how genome instability inhibits the induced transition between different cellular states.