This is a pediatric basket study to investigate the safety and efficacy of afamitresgene autoleucel in HLA-A*02 eligible and MAGE-A4 positive subjects aged 2-17 years of age with advanced cancers.

Inclusion Criteria:

• Subject has histologically confirmed diagnosis of any one of the following cancers: (A) Synovial Sarcoma (SS), (B) MPNST, (C) Neuroblastoma, or (D) Osteosarcoma (OS).
• Age:

(A) Synovial Sarcoma: 2 to 17 years (B) MPNST, Neuroblastoma and Osteosarcoma: 2 to 21 years

• Body weight ≥ 10 kg
• Must have previously received a systemic chemotherapy
• Measurable disease prior to lymphodepletion according to RECIST v1.1 (or INCR, 2017 Neuroblastoma only).
• HLA-A*02 positive
• Tumor shows MAGE-A4 expression confirmed by central laboratory.
• Performance Status:

(A) Subjects ≥16: Eastern Cooperative Oncology Group (ECOG) 0 or 1 (B) Subjects 2 to 16: Lansky score ≥ 80

• Subject has anticipated life expectancy of greater than 3 months in the opinion of the investigator.

Exclusion Criteria:

• Positive for HLA-A*02:05 in either allele; or any A*02 having same protein sequence as HLA-A*02:05
• History of allergic reactions attributed to compounds of similar chemical or biologic composition to fludarabine, cyclophosphamide.
• History of autoimmune or immune mediated disease
• Known central nervous system (CNS) metastases.
• Other prior malignancy that is not considered by the Investigator to be in complete remission
• Clinically significant cardiovascular disease
• Active infection with human immunodeficiency virus, hepatitis B virus, hepatitis C virus, or human T cell leukemia virus
• Pregnant or breastfeeding
• Experiencing ongoing rapid disease progression that in the opinion of the Investigator significantly increases the subjects risk associated with treatment.