Summary The development of the human body involves cell specification and cell fate transition starting from the embryos. Precise coordination of gene expression networks is required: lineage-specific genes are transcriptional activated during early development, while genes for ectopic lineages are repressed in the process. The precise control of gene expression is governed by epigenetic chromatin modifications. Recent advances showed that genes encoding epigenetic “writer” and “eraser” enzymes are frequently mutated in human diseases. However, treatment or early prevention methods are hampered by the lack of knowledge on how the epigenetic landscape is precisely regulated. We are investigating how a critical histone methylation (trimethylation of lysine 27 on histone H3, or H3K27me3) is precisely regulated. H3K27me3 is the hallmark for facultative heterochromatin, which dynamically regulate gene repression during body development. During early differentiation, H3K27me3 is deposited on pluripotency genes, erased on cardiac genes, and maintained on other developmental genes for ectopic lineages. The dynamic level of H3K27me3 across the genome is essential for the ON/OFF switch of gene expression, but it remained unclear how H3K27me3 is regulated in a coordinated temporal and spatial manner. We hypothesize that key macromolecular interactions including protein-protein and protein-nucleic acid interactions regulates the specificity of the H3K27me3 “writer” enzyme – Polycomb Repressive Complex 2 or PRC2. Recent progress and our preliminary data show that the dynamic interactions between PRC2 and its accessory proteins play key roles in the spatiotemporal regulation of epigenetic silencing specificity. To test the hypothesis, we propose to fully interrogate the mechanism by employing a series of separation-of- function mutants. Understanding the mechanism will open to door to further identification of novel therapeutical targets to manipulate gene expression through epigenetic mechanisms.