Polycomb and Trithorax group proteins regulate silent and active gene expression states. but also allow poised states in pluripotent cells. Here the authors present a mathematical model that integrates data on Polycomb/ Trithorax biochemistry into a single coherent framework which predicts that poised chromatin is not bivalent as previously proposed. but is bistable. https://www.roneverhart.com/Oakley-Sutro/