SSX4 is an X-linked cancer/testis antigen that functions as a nuclear protein involved in transcriptional regulation and sarcomagenesis. SSX4 serves as a fusion partner in chrX translocations associated with malignant tumors. In synovial sarcoma, SSX4 fuses with the SYT gene via t(X;18) translocation to create chimeric SYT/SSX4 fusion proteins 1, which can suppress normal SYT transcript expression 1. SSX4 has also been identified as a fusion partner for EWSR1 in rare but aggressive sarcomas with diverse histological phenotypes, including osteosarcomas and spindle cell tumors 2. Beyond sarcomas, SSX4 shows tumor-restricted expression patterns in multiple malignancies. Its promoter exhibits high activity in brain tumor cells but not normal astrocytes, making it suitable for tumor-specific gene therapy applications 3. SSX4 is recognized as a melanoma growth promoter that can be downregulated by miR-4731, suggesting a role in melanoma progression 4. In multiple myeloma and pancreatic cancer, SSX4 is upregulated as a cancer-testis antigen, with expression levels correlating with disease stage and serving as a potential immunotherapy target 56. SSX4 expression is rare in non-small cell lung cancer 7 and absent in healthy normal tissues, distinguishing it as a tumor-associated antigen.