Background: T cells modified to target NY-ESO-1 have shown encouraging activity in HLA-A*02⁺ patients with NY-ESO-1–positive synovial sarcoma. NY-ESO-1 is a cancer/testis antigen that is expressed across multiple tumor types and highly expressed in synovial sarcoma. NY-ESO-1 TCR T (GSK3377794) are autologous polyclonal T cells transduced by a self-inactivating lentiviral vector to express an affinity-enhanced TCR able to recognize NY-ESO-1 epitope in complex with HLA-A*02. Ongoing trials are evaluating GSK3377794 in multiple solid tumors and multiple myeloma. Methods: This study (NCT03967223) uses a Master Protocol design that allows investigation of GSK3377794 in multiple tumor types under the same protocol in separate substudies. The first two are single-arm substudies in patients with advanced metastatic or unresectable synovial sarcoma: treatment-naïve (1^st line [1L], substudy 1; n = 10 planned) and progressing after anthracycline-based chemotherapy (2L+, substudy 2; n = 55 planned). Patients must be aged ≥10 years, have adequate organ function, ECOG performance status 0–1, measurable disease, and no central nervous system metastases. Excluded prior treatments include gene therapy with an integrating vector or NY-ESO-1–specific T cells, vaccine or targeting antibody, or allogeneic stem cell transplant. Patients will undergo leukapheresis and manufacture of GSK3377794; lymphodepletion then GSK3377794 infusion, followed by safety and disease assessments; and long-term follow-up for 15 years (under a separate protocol). The primary objective of substudy 2 is overall response rate per RECIST v1.1 by central independent review. Secondary objectives include time to response, duration of response, disease control rate, progression-free survival, overall survival, plus safety and tolerability. Exploratory objectives include assessment of the correlation of T-cell persistence with safety, clinical responses, and infused T-cell phenotype. Evaluation of quality of life and daily functioning of patients will also be assessed. Enrollment began in December 2019. These data are presented on behalf of the original authors with their permission. A similar presentation (P453) was presented at the SITC Annual Meeting, National Harbor, MD, USA, Nov 6–10, 2019. Funding: GlaxoSmithKline (208467) Clinical trial information: NCT03967223.