The earliest trials of immune checkpoint inhibitors to include STS provided little evidence of activity, with no patients responding in studies of the anti-CTLA-4 mAb ipilimumab and anti-PD1 mAb nivolumab or pembrolizumab in synovial sarcoma (SS) and uterine leiomyosarcoma (LMS), respectively (Table 1) [13,14,15]. This evidence concerns the gene PDCD1 and synovial sarcoma.