Using the HS-SY-II cell line, which has the SS18::SSX1 fusion gene as a positive control [40], we show that both ICR-SS-1 and its originating PDX tumour J000104314 harbour the SS18::SSX1 fusion gene (Figure 2A), confirming the diagnosis of synovial sarcoma. The gene discussed is SS18; the disease is synovial sarcoma.