Chinese scientists reported the first high-resolution cryo-EM structure of human Polycomb repressive deubiquitinase (PR-DUB) complex bound to the chromatosome particle with mono-ubiquitinated H2AK119 (H2AK119ub1), revealing the mechanism by which PR-DUB specifically removes H2AK119ub1 in the context of nucleosome or chromatosome.