17-P038 Sox3 as a transcriptional repressor

immediate early step in PrE specification. We show that the fraction cells present with in this state is influenced by factors that both promote and suppress primitive PrE differentiation, however conditions that support ES cell self-renewal prevent their progression into endoderm differentiation and support an equilibrium between this state and at least one other that resembles the inner cell mass (ICM) of the blastocsyst. Interestingly, while these subpopulations are equivalently and clonally interconvertable under self-renewing conditions, when induced to differentiate both invivo and invitro they exhibit different behaviours. Taken together our data supports a model in which ES cell culture traps a set of interconvertable cell states reminiscent of early stages in blastocyst differentiation that may exist transiently in the early embryo.