 When keeping an ES cell in its undifferentiated state, bivalent domains of DNA are used to silence developmental genes that would activate cell differentiation, while keeping the genes poised and ready to be activated.When an ES cell receives a signal to differentiate into a specified cell lineage, activation of the specific developmental genes are needed for differentiation.The developmental genes needed will be activated and the other genes that are not required for that cell lineage will be repressed through their bivalent domains.