# SS18::SSX fusion (synovial sarcoma)

Source: https://onco.cc/terms/ss18-ssx-fusion/  
OnCo record `ss18-ssx-fusion` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Every true synovial sarcoma carries a fusion between the SS18 gene on chromosome 18 and an SSX gene on the X chromosome; finding it by FISH, RNA sequencing or a newer antibody stain confirms a diagnosis that pathologists otherwise struggle with, and it also flags the tumour for the MAGE-A4 and NY-ESO-1 cell therapies that work almost only in this disease.

## Summary

What is measured: the SS18::SSX fusion from t(X;18)(p11;q11). How: FISH with an SS18 break-apart probe, RT-PCR or targeted RNA sequencing naming the partner (SSX1 in about two-thirds, SSX2 in a third, SSX4 rarely), or immunohistochemistry with the fusion-specific SS18-SSX antibody (about 95 percent sensitive and near 100 percent specific) and the SSX C-terminus antibody; TLE1 staining is a sensitive but less specific screen. The fusion protein hijacks the BAF (SWI/SNF) chromatin-remodelling complex and displaces SMARCB1, which is why the tumours are partly INI1-deficient; a poorly differentiated component and size over 5 cm worsen the outlook. About 70 percent express MAGE-A4 and 80 percent NY-ESO-1, cancer-testis antigens. What a result changes: it settles the diagnosis against MPNST, solitary fibrous tumour, Ewing sarcoma and poorly differentiated carcinoma; synovial sarcoma is more chemosensitive than most sarcomas (ifosfamide with doxorubicin); with MAGE-A4 immunohistochemistry and HLA-A*02 typing it qualifies patients for afamitresgene autoleucel (approved 2024) and NY-ESO-1-directed letetresgene autoleucel; BRD9 degraders and other BAF-directed drugs are in trials. Where it matters: synovial sarcoma.

## Fields

- Kind: Term
- Last checked: 2026-09-17
- Also known as: SS18-SSX; SS18::SSX1; SS18::SSX2; SS18::SSX4; SYT-SSX; SYT::SSX; t(X;18); t(X;18)(p11;q11); SS18-SSX antibody; SS18-SSX immunohistochemistry; synovial sarcoma fusion; TLE1

## Connected records

- terms: [FISH / ISH (in situ hybridisation)](https://onco.cc/terms/fish/), [Gene fusion](https://onco.cc/terms/gene-fusion/), [HLA-A*02:01 restriction](https://onco.cc/terms/hla-a02-restriction/), [Immunohistochemistry (IHC)](https://onco.cc/terms/ihc/)
- technologies: [RNA sequencing & expression profiling](https://onco.cc/technologies/rna-seq/)
- targets: [MAGE-A4](https://onco.cc/targets/mage-a4/)
- drugs: [Afamitresgene autoleucel](https://onco.cc/drugs/afamitresgene-autoleucel/), [Doxorubicin](https://onco.cc/drugs/doxorubicin/), [Ifosfamide](https://onco.cc/drugs/ifosfamide/)
- cancers: [Marginal zone lymphoma](https://onco.cc/cancers/marginal-zone-lymphoma/), [Synovial sarcoma](https://onco.cc/cancers/synovial-sarcoma/)

---
JSON: https://onco.cc/api/v1/entities/ss18-ssx-fusion.json