Project Summary Cell identity is determined by cell specific transcription circuitry that integrate information of cell history and extracellular cues. It often involves multilayered regulation at the level of chromatin. Epigenomic studies have identified conserved chromatin features associated with transcription activation. Among them is enrichment of histone H3 K4 methylation at cis-regulatory elements including gene promoters and enhancers. These regulatory sequences often function as integrated transcription factor binding platforms that have unexpected complexity and dynamics in different cell types. In mammal, H3 K4 methylation is deposited by the mixed lineage leukemia (MLL) family of enzymes. Each MLL plays an essential and non-redundant function during embryonic development. Recurrent mutations in MLLs are reported in many human diseases including developmental disorders, cancers and immunological diseases. However, questions of specific function and regulation of each MLL remain. It is also unclear whether the enzymatic activity of MLLs has any obligatory role in transcription regulation. Here we will examine the function of MLL1, the founding member of the MLL family, in cell fate determination and transition using a robust stem cell differentiation and reprograming model. We will dissect the essential function of each MLL1 domain including the catalytic SET domain in cell fate determination. We will also examine the mechanism by which MLL1 regulates transcription and how it may be distinct from other MLLs. Our study will provide novel insights into basic mechanisms of pluripotency and gene regulation.