We confirm that H2AK119ub1 is important for SS18-SSX specific chromatin targeting6, and further show that in synovial sarcoma, PRC1.1 is central in establishing H2AK119ub1 deposition and orchestrating oncofusion protein occupancy and maintenance, with PCGF1 removal leading to global erosion in SS18-SSX binding. Here, SS18 is linked to synovial sarcoma.