BMI1 and synovial sarcoma: Similarly, using a retroviral system that allowed expression of SYT-SSX2 in U2OS cells (human osteosarcoma) at levels similar to those detected in primary synovial sarcoma cells (Syn1 cells- Figure 1A; in synovial sarcoma, SYT-SSX2 expression is driven by the robustly active promoter of SYT), we were able to detect substantial colocalization of SYT-SSX2 with the polycomb protein Bmi1 (Figure 1B).