CD28 and synovial sarcoma: CD28 on CD39+ secreting CD4 regulatory T cell (OR = 1.096, 95%CI 1.006–1.195, p = 0.036, PFDR = 0.050) was a risk factor for SS, and the results of the other four methods were similar: MR-Egger (OR = 1.070, 95%CI 0.946–1.210, p = 0.295), weighted median (OR = 1.049, 95%CI 0.929–1.183, p = 0.440), simple mode (OR = 1.156, 95%CI 0.953–1.403, p = 0.155), and weighted mode (OR = 1.046, 95%CI 0.933–1.172, p = 0.451).