No junction-specific antibody exists for any KMT2A fusion - and synovial sarcoma shows exactly what one would buy
also called fusion-specific antibody absence · why is there no anti-KMT2A::AFF1 antibody · E9X9V analogy
An antibody raised against the join itself would see the fusion protein and nothing else. One exists for a different cancer - synovial sarcoma - where it replaced genetic testing and works for mapping the fusion onto DNA. Nothing like it has ever been made for any KMT2A leukemia fusion, and our calculation says only about six versions would be needed to cover nearly every patient.
The same thing, for a scientist
Every attribution problem in this beat is caused by antibodies raised against retained sequence. An antibody raised against the **junction** has none of them. This is not hypothetical: **E9X9V**, a monoclonal designed to the SS18-SSX breakpoint, was validated by immunoblot, immunoprecipitation **and ChIP-seq**, captures the fusion on chromatin at established target sites genome-wide, and in 400 tumours gave 95% sensitivity and 100% specificity, with none of 300 histological mimics staining [E1]. The paper's own conclusion is that it could replace molecular genetic testing and that it gives the research community a tool for biochemical and genomic interrogation of the fusion protein. **No equivalent reagent exists for KMT2A::AFF1 or for any other KMT2A fusion.** A PubMed search for fusion-, breakpoint- or junction-specific antibodies returns 9 records, every one of them synovial sarcoma. The usual explanation is that KMT2A breakpoints are distributed while the SS18-SSX breakpoint is near-invariant. That explanation is **quantitatively wrong at the protein level**: mRNA splicing collapses a multi-kilobase genomic breakpoint distribution onto a handful of exon boundaries, and this beat's computation finds exactly **six** distinct in-frame KMT2A::AFF1 protein junctions when AFF1 exon 4 is the acceptor - which it is in 1,261 of 1,262 cases [E2]. Six monoclonals, not thousands.
The six epitopes
With AFF1 exon 4 as the acceptor, the in-frame KMT2A donor boundaries in the breakpoint region give these junction sequences (8 residues either side):
| KMT2A last residue | junction 16-mer |
|---|---|
| 1052 | QTDQPKAQ | QTYSNEVH |
| 1362 | VKQKPKEK | QTYSNEVH |
| 1406 | HRIRVDFK | QTYSNEVH |
| 1444 | CASSGHVE | QTYSNEVH |
| 1493 | GRQHQATK | QTYSNEVH |
| 1525 | PTKKKKVW | QTYSNEVH |
The 1406 row is the architecture of the deposited RS4;11 fusion protein. Note what the table makes obvious and what no genomic breakpoint map shows: every one of these shares the same eight AFF1 residues, so the six epitopes differ only on the KMT2A side. A panel raised against the six KMT2A octamers in the context of QTYSNEVH is a tractable immunisation campaign, not a heroic one.
The same logic applies to the other partners with their own acceptor exons, and the grid for KMT2A::MLLT3 and the reciprocal product is in analyses/junction-peptide-detectability/result.json.
What the SS18-SSX reagent actually bought, item by item
This matters because it is the only worked example, and it shows which of this beat's audit failures a junction antibody fixes.
- Complex composition - fixed. The antibody was validated by immunoprecipitation, so a junction-antibody IP recovers the fusion and not the wild-type parents.
- Chromatin occupancy - fixed. ChIP-seq with the fusion-specific antibody recovered the fusion at established targets genome-wide. This is the capability that no KMT2A experiment has ever had.
- Localisation - fixed. Immunohistochemistry gave strong diffuse nuclear staining in 95 of 100 cases and nothing in 300 mimics, so subcellular distribution is assignable to the fusion.
- Post-translational modification - partly. A junction-antibody IP followed by mass spectrometry is precisely the experiment that would map the first phosphosite on a KMT2A fusion protein, which this graph records as never having been done.
- Half-life - partly. Cycloheximide chase read out with a junction antibody gives fusion turnover directly.
- KMT2A* - not fixed. It has no junction, so a junction antibody cannot see it, ever.
The bounded search
| query | count | content |
|---|---|---|
("fusion-specific antibody" OR "fusion specific antibody" OR "breakpoint-specific antibody" OR "junction-specific antibody")[tiab] | 9 | all synovial sarcoma / SS18-SSX; none leukaemia |
(KMT2A OR MLL)[tiab] AND fusion[tiab] AND ("specific antibody" OR "monoclonal antibody")[tiab] AND (junction OR breakpoint)[tiab] | 1 | a clonal-tracing case report, not an antibody |
"neoepitope"[tiab] AND fusion[tiab] AND (antibody OR antibodies OR "T cell receptor")[tiab] | 6 | fusion neoepitope work is T-cell-directed (NPM1::ALK, DNAJB1-PRKACA vaccine), not antibody-directed |
The third row is worth noting as a live adjacent field: fusion junctions are being exploited as neoepitopes, but for T-cell receptors and vaccines rather than for research reagents. Nobody has taken the obvious step of making the same junction an antibody epitope in leukaemia.
| status | run |
|---|---|
| junction specific antibodies for kmt2a fusions | 0 |
| distinct in frame kmt2a aff1 protein junctions | 6 |
| shared acceptor octamer | QTYSNEVH (AFF1 347-354) |
| why six and not thousands | mRNA splicing collapses the genomic breakpoint distribution onto coding-exon boundaries; the acceptor exon is invariant in 1261 of 1262 cases |
| cannot detect | protein-kmt2a-star-truncated-product-of-the-internal-promoter |
| absence class | never-measured |
| absence class basis | No such reagent has been made, so nothing could have been measured with it. |
| absence class assigned | 2026-09-21 (cycle 47, king) |
worked example in another disease
{
"clone": "E9X9V",
"target": "SS18-SSX breakpoint",
"validated_in": [
"immunoblot",
"immunoprecipitation",
"ChIP-seq",
"immunohistochemistry"
],
"sensitivity_pct": 95,
"specificity_pct": 100,
"n_tumours": 400,
"pmid": "32141887",
"title": "A Novel SS18-SSX Fusion-specific Antibody for the Diagnosis of Synovial Sarcoma.",
"authors": "Baranov E, McBride MJ, Bellizzi AM, et al.",
"venue": "Am J Surg Pathol",
"year": 2020,
"title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
}junction epitopes 16mer
[ "QTDQPKAQ|QTYSNEVH", "VKQKPKEK|QTYSNEVH", "HRIRVDFK|QTYSNEVH", "CASSGHVE|QTYSNEVH", "GRQHQATK|QTYSNEVH", "PTKKKKVW|QTYSNEVH" ]
queries
[
{
"query": "(\"fusion-specific antibody\"[tiab] OR \"fusion specific antibody\"[tiab] OR \"breakpoint-specific antibody\"[tiab] OR \"junction-specific antibody\"[tiab])",
"records": 9,
"note": "all synovial sarcoma"
},
{
"query": "(KMT2A[tiab] OR MLL[tiab]) AND fusion[tiab] AND (\"specific antibody\"[tiab] OR \"monoclonal antibody\"[tiab]) AND (junction[tiab] OR breakpoint[tiab])",
"records": 1,
"note": "a clonal-tracing case report, not an antibody"
}
]- A diagnostic junction antibody covering six epitopes would miss patients whose breakpoint lies outside them, and those patients are enriched for complex rearrangements - so a negative result would be systematically biased towards a group with distinct biology and would look like a true negative.
- If a junction antibody became the standard research reagent, historical results obtained with retained-region antibodies would not be directly comparable, and the field would face a decade of results it could neither reproduce nor discard.
- Is the KMT2A::AFF1 junction immunogenic? The KMT2A side of the commonest epitope is HRIRVDFK - charged, exposed and in a disordered region, which is favourable - but nobody has tried, and a failed immunisation campaign would itself be worth knowing about.
- Would a junction antibody raised against KMT2A 1406 | AFF1 347 cross-react with the wild-type parents? The epitope is absent from the reviewed human proteome as a linear 16-mer, but antibodies recognise conformations, and both halves individually are present in every cell.
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
A monoclonal antibody designed to the SS18-SSX breakpoint (E9X9V) was validated by immunoblot, immunoprecipitation and ChIP-seq, captures the fusion on chromatin at established target sites genome-wide, and gave 95% sensitivity and 100% specificity across 100 synovial sarcomas and 300 histological mimics.
A Novel SS18-SSX Fusion-specific Antibody for the Diagnosis of Synovial Sarcoma. · Baranov E, McBride MJ, Bellizzi AM, et al. · Am J Surg Pathol 2020What this evidence cannot show
Abstract read verbatim 2026-09-20. SS18-SSX is a far easier target: the breakpoint is near-invariant, the fusion is abundant, and the tumour is not a blood cancer with a wild-type partner in every cell. The paper does not establish that the same approach transfers to a nuclear low-abundance protein in a haematopoietic cell. It establishes that the approach exists and that ChIP with a junction antibody works.
Evidence that argues against this (1)
- The companion antibody in the same paper, raised against the SSX C-terminus rather than the breakpoint, achieved 100% sensitivity but stained 13 of 300 control tumours - a direct demonstration that a retained-region antibody is less specific than a junction antibody even in the easy case, and a reason not to expect a retained-region reagent to suffice in the hard one. Baranov E et al. 2020
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
The AFF1 acceptor exon is essentially invariant - the genomic break lies upstream of AFF1 exon 1 in 1 of 1,262 KMT2A::AFF1 cases - so the protein-level junction varies only on the KMT2A side, giving six distinct in-frame epitopes rather than a continuous distribution.
The KMT2A recombinome of acute leukemias in 2023. · Meyer C, Larghero P, Almeida Lopes B, et al. · Leukemia 2023What this evidence cannot show
The 1-of-1,262 figure is this graph's own corrected reading of the recombinome and replaces an 'about half' figure that belonged to MLLT1. The six-epitope count is computed here from RefSeq coding-exon boundaries, not taken from the paper; intraexonic genomic breakpoints and phase-1/2 donors that pair with a different acceptor exon would add further epitopes, so six is a floor for the common architecture rather than an exhaustive count.
Evidence that argues against this (1)
- Breakpoints outside the main cluster region, intraexonic breaks, and the complex rearrangements that make up a substantial minority of cases would each generate epitopes outside the six, so a six-antibody panel would have a real and non-random miss rate - and the missed patients would be enriched for the complex-rearrangement group that already has distinct biology. Meyer C et al. 2023
- E3moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
Fusion junctions are being actively exploited as neoepitopes in leukaemia and other cancers, but by T-cell receptors and peptide vaccines rather than by antibodies, so the junction-as-epitope idea is current in the field and has simply not been applied to make a research or diagnostic antibody for KMT2A.
Endogenous CD4+ T Cells That Recognize ALK and the NPM1::ALK Fusion Protein Can Be Expanded from Human Peripheral Blood. · Stadler S, Blasco RB, Singh VK, et al. · Cancer Immunol Res 2025What this evidence cannot show
Cited only for the existence and currency of the junction-as-epitope strategy in a fusion-driven haematological malignancy. The T-cell work depends on MHC presentation, which is a different and in some ways harder requirement than antibody recognition, so it neither establishes nor excludes that an anti-KMT2A-junction antibody is raisable.
Evidence that argues against this (1)
- A fusion-derived peptide vaccine programme has reached clinical trial for DNAJB1-PRKACA, which shows the junction epitope route is being pursued where the junction is invariant - and the KMT2A junction's variability is exactly the property that has kept it out of such programmes. Hackenbruch C et al. 2024
a knock-in tag is the way to get fusion specificity without a junction antibody
the absence of a specific antibody is survivable because specificity can live in the readout instead
the attribution failures of every retained-sequence reagent are what a junction antibody would remove
a junction epitope is the one reagent class that can separate fusion from wild-type and from KMT2A*
the same epitope argued for therapeutically, here argued for as a research reagent
the mass-spectrometry route to the same attribution problem, which needs no new reagent
the cheaper experiment available today while no junction antibody exists
supplies the missing precedent: a junction-raised monoclonal against a paediatric leukaemia fusion, ChIP-capable, which the entry's synovial-sarcoma exemplar could not demonstrate
SURVIVES in the Antibody Registry under five fusion spellings with the E9X9V clone as a same-template positive control; two leukaemia-fusion junction reagents found in the process
the experiment that would close this absence, using a protocol published for another paediatric leukaemia fusion
SURVIVES a reagent-vocabulary re-query; the nearest object is a two-antibody immunobead sandwich
a 1997 paper reaching the same conclusion for diagnostic immunocytochemistry