The key transcription factors that control the embryonic stem cell gene expression program have been identified, but how they function to implement this program is not well understood. While screening for genes essential for maintenance of ES cell state, we identified many components of the Mediator and Cohesin complexes. Mediator and Cohesin were found to physically and functionally connect the enhancers and core promoters of active genes. An ES cell Mediator complex was found to copurify with Cohesin and its loading factor Nipbl, and normal levels of these proteins were essential for expression of the genes they occupy and for maintenance of ES cell state. See associated publication.