Step 4: the split-signal FISH nucleus - one fusion, one green, one red
also called FISH · break-apart FISH · split-signal FISH · dual-colour break-apart probe · KMT2A break apart · MLL break apart · 1F1G1R · 2F
Two coloured probes are stuck to the chromosome either side of the place KMT2A usually breaks, so an intact gene shows them merged as one dot and a broken gene shows them flown apart as two. Three dots instead of two is the diagnosis.
The same thing, for a scientist
Dual-colour break-apart (split-signal) FISH is the workhorse confirmatory assay for KMT2A rearrangement. Two differentially labelled probe cocktails hybridise to sequence flanking the KMT2A breakpoint cluster region at 11q23.3 - one proximal/5', one distal/3'. In an intact allele the two probe footprints lie within a few hundred kilobases of each other, far below the resolving power of light microscopy in an interphase nucleus, so their emissions are integrated into a single colocalised (fusion) signal; a normal nucleus therefore scores two fusion signals, 2F [E1]. When KMT2A is disrupted and its 3' portion translocates to the partner chromosome, the two probes on that allele occupy different chromosome territories and resolve into separate dots: the canonical rearranged pattern is one fusion, one green, one red - 1F/1G/1R - with the fusion signal contributed by the intact allele [E1][E2]. The assay works on interphase nuclei and therefore needs no cell division, which is why it recovers cryptic insertions and translocations that a G-banded karyotype misses [E3][E4]. Its essential limitation is definitional: break-apart FISH establishes that KMT2A is broken and says nothing about which of the 100-plus partner genes it joined [E5].
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Two probe cocktails are hybridised to sequence flanking the KMT2A breakpoint cluster region at 11q23.3, one proximal (5') and one distal (3'), each carrying a different fluorophore.
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In an intact allele the two probe footprints are separated by far less than the resolving power of light microscopy in an interphase nucleus, so their point-spread functions overlap and the detector integrates them into one spot whose colour is the additive mixture - green plus orange reads as yellow. Two intact alleles therefore give 2F.
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When KMT2A breaks and its distal portion translocates - to 4q21 in t(4;11) - the two probes on that allele end up on different chromosomes occupying different interphase chromosome territories, far enough apart to resolve as two separate dots. The unrearranged allele still fuses. Hence 1 fusion / 1 green / 1 red.
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Colour conventions differ between manufacturers: some label the proximal side green and the distal side orange, others the reverse. The pattern grammar (2F normal, 1F+1G+1R broken) is universal; the colour-to-strand mapping is not, and must be read from the specific probe's documentation.
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A pattern of 1F plus only one separate colour means the allele broke AND the sequence covered by the missing probe was not retained, through loss of the derivative partner chromosome or a genuine deletion. It is still a positive result, and because it contains two dots - the same count as a normal nucleus - it is the pattern most vulnerable to being scored as normal.
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Scoring is statistical: roughly 100-200 interphase nuclei are counted and compared against an abnormal-cell cut-off that each laboratory validates on normal controls, because truncated nuclei and single-probe hybridisation failure guarantee a nonzero false-positive rate in every normal sample.
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What this image fundamentally cannot show is the identity of the partner. A 1F/1G/1R nucleus is equally consistent with any of the 107 catalogued in-frame KMT2A fusions. Naming the partner and the exon junction requires RT-PCR, LDI-PCR or targeted RNA sequencing.
Reading the slide, dot by dot
Two fluorescent DNA probes are stuck onto the chromosome either side of the place where KMT2A usually breaks. If the gene is intact the two colours sit so close together that the microscope cannot separate them and you see one blended yellow dot per copy of the gene - two yellow dots. If KMT2A has been broken and half of it has moved to another chromosome, that copy's two colours physically fly apart and you see one green dot and one red dot, plus the surviving yellow dot from the unbroken copy. Three dots instead of two is the diagnosis.
The optics: why two dots become three2 deeper
This is a physics story before it is a genetics story.
A break-apart probe set is two cocktails of labelled DNA, each typically hundreds of kilobases long, complementary to sequence on either side of the KMT2A breakpoint cluster region at 11q23.3. One hybridises proximal to the gene (the centromeric, 5' side), the other distal (the telomeric, 3' side). Each carries a different fluorophore. On the MetaSystems XL MLL plus probe, for example, the proximal probe is orange-labelled and the distal probe green-labelled 1.
Now consider what the microscope can actually resolve. In a fixed interphase nucleus, two point sources of light separated by less than roughly a megabase of chromatin cannot be told apart: their point-spread functions overlap and the detector integrates them into one spot whose colour is the additive mixture of the two dyes - green plus orange/red reads as yellow. Because the two probes flank a gene that is only about 90 kb long, in an intact allele they are always inside that unresolvable distance. So:
- Intact KMT2A, both alleles -> two colocalised green-orange signals. Reported as 2 fusion (2F), or 2GO in the vendor's notation 1.
Now break the gene. In t(4;11)(q21;q23) the 3' portion of KMT2A translocates to chromosome 4q21, joining AFF1. The distal probe goes with it. The two probes that were previously a few tens of kilobases apart are now on two different chromosomes, which in interphase occupy separate chromosome territories tens of nanometres to micrometres apart - comfortably resolvable. The other allele is untouched and still fuses. So:
- KMT2A rearranged, one allele -> one colocalised fusion signal (from the intact allele), plus one separate green and one separate orange/red signal (from the split allele). Reported as 1 fusion / 1 green / 1 red (1F/1G/1R), or 1GO+1G+1O 1. The case-report literature states the same thing in clinical language: 'when there is an MLL gene translocation, the usual FISH signal pattern is 1 red-1 green-1 yellow fusion signal pattern' 2.
The whole assay is that one idea: proximity is the read-out. Nothing about the fusion protein, the transcript, or the partner is being measured. What is being measured is whether two pieces of DNA that belong together are still next to each other.
Do not memorise which colour means 3'
Colour conventions are vendor-specific, and getting this backwards is a real reporting error. Some kits label the 5'/proximal side green and the 3'/distal side orange or red; the MetaSystems XL MLL plus probe does the reverse, with orange proximal and green distal 1. Historical Vysis-style nomenclature commonly pairs SpectrumGreen with SpectrumOrange, which is why laboratory reports say 'orange' where the literature and this node say 'red'.
The practical rule: a split-signal report must be read against the specific probe's stated expected patterns, not against a remembered colour-to-strand mapping. What is universal across vendors is the pattern grammar - two fused signals is normal, one fused plus two separated is a break - not the colour assignment.
What the scoring actually consists of
Interphase FISH is a counting assay, and its interpretation is statistical rather than binary. A slide of the diagnostic marrow is hybridised (conventionally overnight, with rapid protocols shortening this), counterstained with DAPI, and nuclei are scored - typically on the order of 100 to 200 interphase nuclei, by two independent observers or an automated scanner.
Every normal sample contains some nuclei that score falsely abnormal. Truncated nuclei in the preparation lose one signal; random hybridisation failure of one probe produces an apparent single-colour signal; two neighbouring nuclei overlap and appear to have extra signals. Laboratories therefore establish an abnormal-cell cut-off by scoring a series of normal control samples and setting the threshold above the observed false-positive rate, and a result below that locally validated cut-off is reported as normal rather than as a low-level positive. That cut-off is a property of the laboratory, the probe lot and the preparation quality, not of the biology - which is why an inter-laboratory comparison of 'positive' at low percentages is not meaningful without the cut-offs attached.
Metaphase FISH, where available, is more interpretable than interphase FISH for the same probe, because the signals can be assigned to identified chromosomes: in a documented t(9;11) case metaphase FISH resolved three distinct cell populations that interphase FISH collapsed into two [E3, see counter-evidence].
The pattern grammar: what each result means
| Pattern | Notation | Interpretation |
|---|---|---|
| Two colocalised signals | 2F (2GO) | both KMT2A alleles intact, or a rearrangement outside the probe footprint 1 |
| One fusion, one green, one red | 1F/1G/1R | canonical balanced disruption of one KMT2A allele; the standard positive result 12 |
| One fusion, one green only (3' or distal signal absent) | 1F/1G | the allele broke and the sequence covered by the missing probe was not retained - either the derivative partner chromosome carrying it was lost, or there is a genuine deletion distal to the breakpoint. Still a positive result, and easy to under-call |
| One fusion, one red only | 1F/1R | the mirror case; loss of the sequence covered by the other probe |
| Three or more fusion signals | 3F, 4F | insertion, duplication of the locus, or polyploidy; needs metaphase FISH or an orthogonal assay to disentangle |
| Two fusions, one attenuated or diminished | 2F with a small signal | partial deletion inside the probe footprint. The analogous phenomenon is documented for the RARa break-apart probe, where a dual-colour break-apart probe revealed a partial deletion of RARa in a case that conventional cytogenetics and RT-PCR had not resolved 5 |
The deletion patterns are the ones to worry about. A 1F/1G or 1F/1R nucleus contains only two dots, the same number as a normal 2F nucleus. If a scorer is counting signals rather than reading colocalisation, a deleted-signal rearrangement can be logged as normal. This is the most common way a break-apart FISH result is quietly wrong in the positive-to-negative direction.
Atypical patterns are not rare enough to ignore. A published infant case of CALLA-negative precursor B-ALL with a characteristic t(4;11)(q21;q23) presented with an unusual MLL FISH signal pattern, not the textbook 1R/1G/1F 2. Any pattern that is not clean 2F or clean 1F/1G/1R is a trigger for a second method, not for interpretation by inference.
What FISH buys you over karyotype - and what it still cannot tell you
The gains are real and specific.
- No cell division required. Interphase FISH reads non-dividing nuclei, which dissolves the karyotype's worst failure mode. In a 51-infant series, 13 (25%) had contradictory cytogenetic and Southern-blot results; FISH with an MLL probe resolved five of these as normal dividing cells coexisting with non-dividing leukaemic cells, and five as cryptic insertion or translocation, with subsequent work naming MLL-AF10, MLL-AF4 or MLL-AF1q fusions in four 3.
- Cryptic rearrangements become visible. An insertion that leaves the band pattern untouched still separates the two probes. This is the single strongest argument for running FISH in every infant regardless of what the karyotype says 34.
- Speed and specimen tolerance. An overnight hybridisation on a marrow or blood slide, with no culture step, puts the answer roughly a day ahead of the karyotype.
- It is the assay international guidance names as a reliable alternative detection method for the prognostically decisive abnormalities in B-cell precursor ALL, including t(4;11)(q21;q23)/MLL-AFF1 4.
The limits are structural, not fixable by better technique.
- It cannot name the partner. A 1F/1G/1R nucleus is equally consistent with AFF1, MLLT1, MLLT3, MLLT10, ELL, MLLT4 or any of the long tail. In 3401 patients characterised at the genomic breakpoint, 107 in-frame KMT2A fusions have been catalogued, 37 recurring and 63 seen only once 5. The image that defines this disease visually encodes exactly the one fact it cannot deliver.
- It cannot see the exon junction, so it cannot define the retained and lost domains, and it cannot supply the patient-specific sequence that quantitative MRD needs - see
concept-mrd-monitoring-genomic-breakpointandconcept-molecular-confirmation-fusion-transcript. - It cannot distinguish a reciprocal rearrangement from a der(11)-only product, which the recombinome shows is a real distinction: of 3401 patients, 18 had no partner gene fused to 5'-KMT2A at all and two had a 5'-KMT2A deletion 5.
- It is blind inside its own footprint. A KMT2A partial tandem duplication does not separate two flanking probes, so a PTD reads as 2F.
- It is not a recovery tool when the prior is low. In 38 AML cases with no 11q23 translocation on conventional cytogenetics, interphase FISH found MLL rearrangement in none, and the authors concluded interphase FISH 'seemed less useful for the detection of MLL rearrangement' than for AML1/ETO, PML/RARa or CBFB/MYH11 [E5 counter-evidence; primary source cited there]. FISH rescues the karyotype in infants with a strong pro-B/NG2 prior; it does not manufacture diagnoses from unselected normal karyotypes.
| probe type | dual-colour break-apart |
|---|---|
| locus | 11q23.3 |
| gene | KMT2A |
| normal pattern | 2 fusion |
| rearranged pattern | 1 fusion, 1 green, 1 red |
| colour convention warning | Which colour marks the 5'/proximal versus 3'/distal side is vendor-specific and reverses between kits; read the probe's stated expected patterns, not a remembered mapping. |
| physical basis | two flanking probes on an intact allele lie well below the resolution limit of light microscopy in an interphase nucleus, so their emissions colocalise into a single blended (yellow) signal; a translocation moves them onto different chromosomes and therefore into different interphase chromosome territories, resolving them into separate dots |
| specimen | bone marrow or peripheral blood smear or cytospin; fixed cells, no culture required |
| requires cell division | false |
| hybridisation | conventionally overnight; rapid protocols shorter |
| cut off | laboratory-validated against a series of normal controls; a result below the local cut-off is reported as normal. The cut-off is a property of the laboratory, probe lot and preparation quality, not of the biology |
vendor notation example
{
"probe": "MetaSystems XL MLL plus (D-5060-100-OG)",
"normal": "2GO",
"rearranged": "1GO, 1G, 1O",
"proximal_label": "orange",
"distal_label": "green"
}interphase nuclei scored
[ 100, 200 ]
patterns
[
{
"notation": "2F",
"signals": [
"fusion",
"fusion"
],
"interpretation": "normal, or a rearrangement outside the probe footprint (e.g. KMT2A-PTD)"
},
{
"notation": "1F/1G/1R",
"signals": [
"fusion",
"green",
"red"
],
"interpretation": "canonical balanced KMT2A disruption of one allele"
},
{
"notation": "1F/1G",
"signals": [
"fusion",
"green"
],
"interpretation": "break plus loss of the sequence under the missing probe; positive but easily under-called"
},
{
"notation": "1F/1R",
"signals": [
"fusion",
"red"
],
"interpretation": "mirror image of the above"
},
{
"notation": "3F or more",
"signals": [
"fusion",
"fusion",
"fusion"
],
"interpretation": "insertion, duplication or polyploidy; requires metaphase FISH or an orthogonal assay"
},
{
"notation": "2F with attenuated signal",
"signals": [
"fusion",
"diminished fusion"
],
"interpretation": "possible partial deletion within the probe footprint"
}
]detects
[ "balanced reciprocal translocations involving KMT2A", "cryptic insertions and translocations invisible to G-banding", "rearrangement in non-dividing cells" ]
does not detect
[ "the identity of the 3' partner gene", "the exon/intron junction", "KMT2A partial tandem duplication", "which derivative chromosome carries the expressed fusion" ]
downstream
[ "concept-molecular-confirmation-fusion-transcript", "concept-mrd-monitoring-genomic-breakpoint" ]
- A rearrangement with loss of one flanking signal produces a two-dot nucleus, the same count as normal. A scorer counting dots rather than reading colocalisation will report a high-risk infant as KMT2A-germline, and nothing downstream will contradict it.
- The probe is blind inside its own footprint: a KMT2A partial tandem duplication separates nothing, so it reads as a clean 2F normal result. A negative break-apart FISH is not a negative KMT2A test.
- Break-apart FISH is deliberately partner-agnostic, which makes it the ideal screen and a poor basis for anything downstream that depends on the partner - menin-inhibitor trial eligibility, domain-retention reasoning, or a partner-specific prognosis.
- FISH sensitivity is on the order of a few per cent of nuclei, orders of magnitude worse than PCR. It cannot be the relapse monitor, so a diagnostic pathway that stops at FISH has no MRD target and has spent the one diagnostic marrow that could have provided one.
- Atypical signal patterns occur in cases with an otherwise textbook t(4;11), so interpreting an unexpected pattern by inference rather than by running a second method risks a wrong call on a case that looked characteristic in every other respect.
- Interphase FISH is not a universal rescue: in unselected normal-karyotype AML it recovered no MLL rearrangements at all. Treating it as a safety net rather than as a prior-dependent test will produce false confidence in negative results.
- This nucleus is the project's visual signature, and that itself is a trap: the image most strongly associated with the disease encodes exactly the one thing the assay cannot determine - which partner gene KMT2A joined. Any communication built on it has to say so explicitly or it will teach the wrong thing.
- What is the real-world false-negative rate of a commercial KMT2A break-apart probe when benchmarked against whole-transcriptome or targeted RNA sequencing in a consecutive infant cohort? The literature reports individual discordant cases and one 2003 series with Southern blot as comparator, but no modern head-to-head denominator.
- How often does a deleted-flanking-signal pattern (1F/1G or 1F/1R) get scored as normal in routine practice? Because that pattern has the same signal count as a normal nucleus, the error is silent and would not show up in any audit that compares positive rates.
- Are locally validated interphase cut-offs transferable between laboratories, and does the variation between them change risk-group assignment for any infant at the margin?
- Is there a probe design - three-colour, or a probe internal to the breakpoint cluster region - that would distinguish a reciprocal rearrangement from a der(11)-only product, or flag a partial tandem duplication, without adding a separate assay?
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.database
A dual-colour KMT2A break-apart probe consists of one probe hybridising proximal to the KMT2A gene region at 11q23.3 and one hybridising distal to it, labelled in two different colours; the expected pattern in normal cells is two green-orange colocalisation (fusion) signals (2GO), and in aberrant cells one colocalisation signal plus one separate green and one separate orange signal (1GO, 1G, 1O) resulting from a chromosome break at the locus.
XL MLL plus - Break Apart Probe (product documentation, D-5060-100-OG) · MetaSystems Probes · MetaSystems Probes product catalogue 2026What this evidence cannot show
Manufacturer product documentation, not peer-reviewed literature, and specific to one vendor's probe: the colour-to-side assignment (orange proximal, green distal) is NOT universal, and other kits reverse it. The page does not state the kilobase size of each probe, so the 'few hundred kilobases' figure in the body text is an inference from probe design conventions rather than a quoted specification. Expected-pattern statements from a manufacturer describe intended performance, not measured sensitivity in a clinical cohort.
Evidence that argues against this (1)
- Vendor-stated expected patterns are the ideal case. Real cases deviate: an infant with CALLA-negative precursor B-ALL and a characteristic t(4;11)(q21;q23) showed an unusual MLL FISH signal pattern rather than the expected one, which means the stated grammar cannot be applied mechanically. Devi SG et al. 2011
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In infants with CALLA (CD10)-negative B lymphoblastic leukaemia, KMT2A/MLL rearrangements at 11q23 are most often detected by conventional karyotyping but FISH with a dual-colour break-apart probe is used to identify cryptic translocations, and when an MLL translocation is present the usual FISH signal pattern is one red, one green and one yellow fusion signal; atypical patterns nonetheless occur in cases with a characteristic t(4;11)(q21;q23).
CALLA negative precursor B lymphoblastic leukemia with MLL gene translocation and an unusual FISH signal pattern · Devi SG, Goyal M, Ramakrishna NVSS, Murthy SS · Indian Journal of Pathology & Microbiology 2011What this evidence cannot show
A single case report, so the statement of the usual pattern is a textbook assertion inside a case discussion rather than a measured frequency, and the atypical pattern described is n=1. It does establish that the 1R/1G/1F grammar is the clinically taught expectation and that exceptions reach publication, which is the use made of it here.
Evidence that argues against this (2)
- The "usual" break-apart pattern fails exactly where it matters most. A cryptic t(4;11) can arise by INSERTION of 5' KMT2A sequences into chromosome 4, which does not separate the probes and can be read as normal; conventional karyotyping misses it too. The entry's own framing — FISH as the backstop for cryptic translocations — is the case this exception breaks. Leukemia 2001
- Infant acute leukaemia specifically produces discrepancies between karyotype and molecular studies, resolved only when FISH is applied to non-dividing cells — because infant blasts index poorly and the abnormal clone can be absent from metaphases altogether. A normal karyotype in an infant is uninformative rather than reassuring. Leukemia 2003
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
FISH with an MLL probe resolved the mechanism in 13 of 51 infants with acute leukaemia whose cytogenetic and Southern-blot results for 11q23/MLL rearrangement disagreed, attributing five to normal dividing cells coexisting with non-dividing leukaemic cells in the same marrow and five to cryptic insertion or translocation, with MLL-AF10, MLL-AF4 or MLL-AF1q fusions subsequently identified in four of the latter; the authors concluded FISH should complement cytogenetic analysis whenever cytogenetic and molecular findings are contradictory in infant leukaemia.
Cryptic insertion and translocation or nondividing leukemic cells disclosed by FISH analysis in infant acute leukemia with discrepant molecular and cytogenetic findings · Watanabe N, Kobayashi H, Ichiji O, Yoshida MA, Kikuta A, Komada Y, Sekine I, Ishida Y, Horiukoshi Y, Tsunematsu Y, Yano M, Nakadate H, Kaneko Y · Leukemia 2003What this evidence cannot show
51 infants from one collaboration in 2003; Southern blot was the molecular reference and has its own blind spots. The FISH probe used was a locus-spanning MLL probe rather than a modern commercial dual-colour break-apart kit, so the signal patterns reported are not directly the 2F/1F1G1R grammar described in this node. In two of the thirteen discordant patients the discrepancy was never resolved by any method.
Evidence that argues against this (1)
- Interphase FISH with an MLL probe can be materially harder to interpret than metaphase FISH with the same probe. In a documented AML with t(9;11), metaphase FISH distinguished three cell populations (normal with two signals, abnormal with three signals, abnormal with two signals) whereas interphase FISH collapsed these into only two populations, and the authors concluded that interphase FISH 'may be problematic' for monitoring. Gallo JH et al. 1999
- E4established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.review
For the genetic abnormalities with the most significant impact on treatment and management of childhood B-cell precursor ALL - including t(4;11)(q21;q23)/MLL-AFF1 used for high-risk stratification - cytogenetics and particularly fluorescence in situ hybridisation provide reliable detection methods, chosen according to the preferred technical approach within each treatment protocol.
Detection of prognostically relevant genetic abnormalities in childhood B-cell precursor acute lymphoblastic leukaemia: recommendations from the Biology and Diagnosis Committee of the International Berlin-Frankfurt-Munster study group · Harrison CJ, Haas O, Harbott J, Biondi A, Stanulla M, Trka J, Izraeli S · British Journal of Haematology 2010What this evidence cannot show
Consensus recommendations from 2010, predating routine RNA sequencing and optical genome mapping, and addressed to childhood B-cell precursor ALL as a whole rather than to infants specifically. The document deliberately leaves the choice of primary method to each protocol, so it endorses FISH as adequate without quantifying its false-negative rate, and it does not prescribe interphase scoring numbers or cut-offs.
Evidence that argues against this (1)
- "Reliable detection" is a statement about the common rearrangements a review is written around. Complex and cryptic KMT2A rearrangements — three-way translocations, insertions, and deletions of the 3' portion — are documented as a recurring diagnostic problem, and the choice of technique by local preference means the sensitivity is not a fixed property of the disease. Blood Cells, Molecules and Diseases 2010
- E5established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
A break-apart FISH result establishes that KMT2A is disrupted but not which partner gene is involved: across 3401 acute leukaemia patients, 107 in-frame KMT2A fusions have been catalogued, of which 37 occur recurrently and 63 have been seen only once, while 18 patients had no partner gene fused to 5'-KMT2A and two had a 5'-KMT2A deletion - distinctions a two-probe proximity assay cannot make.
The KMT2A recombinome of acute leukemias in 2023. · Meyer C, Larghero P, Almeida Lopes B, et al., Marschalek R · Leukemia 2023What this evidence cannot show
A referral-based diagnostic series over 20 years, so the partner catalogue reflects which samples were sent for LDI-PCR characterisation, not a population distribution. The paper's subject is molecular characterisation and it does not itself report FISH performance, so its use here is to bound what a break-apart result leaves undetermined rather than to measure FISH sensitivity.
Evidence that argues against this (1)
- FISH does not reliably recover KMT2A rearrangements from unselected normal-karyotype cases. In 38 AML cases with no 11q23 translocation on conventional cytogenetics, interphase FISH found no MLL rearrangement at all, and the authors concluded interphase FISH seemed less useful for detecting MLL rearrangement than for AML1/ETO, PML/RARa or CBFB/MYH11 - the same study also showed a dual-colour break-apart RARa probe revealing a partial deletion, illustrating how deletion patterns arise. Bai SX et al. 2011
argues for genomic rather than RNA confirmation of any fusion that changes management
how the broken gene is actually seen in the diagnostic lab
- related to →MRD by genomic breakpoint: the only assay in this disease that is fusion-specific by construction
FISH cannot supply the patient-specific sequence that quantitative MRD needs
KMT2A rearrangement confirmed by FISH with 5' on chromosome 11 and 3' on chromosome 19
KMT2A rearrangement detected in 93% of interphase nuclei
KMT2A::ELL fusion detected in only 23.6% of analysed cells, a subclonal fraction at transformation