Targeting polycomb to pericentric heterochromatin in embryonic stem cells reveals a role for H2AK119u1 in PRC2 recruitment by Cooper, S, Dienstbier, M, Hassan, R, Schermelleh, L, Sharif, J, Blackledge, N, De Marco, V, Elderkin, S, Koseki, H, Klose, R, Heger, A, Brockdorff, N