Cri du Chat Syndrome: Cytogenetics, the 5p15 Critical Regions, and What iFISH Actually Adds
Published 2026-08-20
Paris, 1963. A paediatrician walks past a nursery and stops, because one infant does not sound like the others. The cry is high, thin, and monotone, pitched somewhere around a kilohertz, and every person who hears it reaches for the same comparison: it sounds like a kitten.
The paediatrician is Jérôme Lejeune. Four years earlier he had been part of the team that found an extra chromosome 21 behind Down syndrome, the first time a human clinical syndrome had ever been pinned to a specific chromosome. So when he encountered these children, he did the thing he now knew how to do. He went back to the karyotype and counted.
This time nothing was extra. Something was missing.
Lejeune, Marie-Odile Lafourcade and Raymond Turpin reported three cases of a partial deletion of the short arm of chromosome 5, and gave the condition the name that stuck: cri du chat, the cry of the cat. It was one of the first demonstrations that losing a piece of a chromosome, rather than gaining a whole one, could produce a recognisable human syndrome. That idea is now so routine it is invisible. In 1963 it was new.
The short version. Cri du chat syndrome is a deletion of the short arm of chromosome 5, written
5p-. It affects roughly 1 in 15,000 to 1 in 50,000 live births, which makes it one of the most common terminal deletion syndromes in humans. Most cases are new events that nobody caused. A minority are inherited from a parent whose own chromosomes are perfectly balanced.
What is actually missing
The deletion removes part of 5p, most often the terminal portion running from band 5p15 out to the telomere. Deletion size is not fixed and this is the single most important thing to understand about the condition: reported deletions range from a few hundred kilobases to more than 40 megabases. Two children with the same three-letter diagnosis can be missing wildly different amounts of chromosome.
Rotate the model below and take the tip off. The left chromosome is a normal 5. The right one loses everything from 5p15.3 outward, which is the classic configuration.
[[chromosome-3d]]
Two neighbouring bands, two different problems
Here is the finding that made cri du chat genuinely interesting to geneticists rather than merely tragic. In the 1990s, work led by Joe Overhauser and colleagues correlated deletion breakpoints against clinical features across large patient series, and the features did not travel together. They mapped to different places.
| Region | What its loss contributes | Genes of interest |
|---|---|---|
5p15.3 (often narrowed to 5p15.31) |
The characteristic high-pitched cry. Deletions confined to this region can produce the cry with comparatively mild cognitive involvement. | TERT sits at 5p15.33 and is hemizygous in most deletions, though its contribution to the phenotype remains debated. |
5p15.2 |
Severe intellectual disability, microcephaly, and the developmental delay that dominates long-term outcome. | CTNND2 (delta-catenin), expressed in neurons and involved in dendritic architecture. SEMA5A, an axon guidance molecule. |
5p15.32 to 5p15.33 |
Speech and language delay maps here in several series, partially separable from general cognition. | Region assignments vary between studies. Treat the boundaries as approximate. |
So the cry and the intellectual disability are not the same finding with a shared cause. They are two separate consequences of two adjacent pieces of DNA, and how much of each a given patient loses shapes the clinical picture. A larger deletion extending proximally into 5p14 and beyond generally means a more severe phenotype.
Worth memorising. 5p15.3 gives you the cry. 5p15.2 takes the development. If a question stem describes a cat-like cry with relatively preserved cognition, it is pointing at a small terminal deletion. If it emphasises profound intellectual disability and microcephaly, the deletion reaches 5p15.2 or beyond.
The cry itself
The name is not a metaphor invented for a textbook. The cry is a real, reproducible acoustic finding, and it is one of the few syndromes where the presenting sign is something you hear rather than something you see. It arises from the larynx and its innervation: a small, narrow, sometimes floppy larynx, an abnormally curved epiglottis, and an underlying neurological contribution to how the whole apparatus is controlled.
Two clinically useful details. First, the cry is highest yield in the newborn period and typically fades over the first two years of life, so it is a neonatal clue rather than a lifelong marker. Second, it is not universal. Its absence does not exclude the diagnosis.
[[cdc-cry]]
Where the deletion comes from
This is the part that matters most in the counselling room, and the part exam questions love.
| Mechanism | Share of cases | What it means for the family |
|---|---|---|
| De novo deletion | roughly 80% to 90% | A new event in gamete formation or very early development. Neither parent carries anything. Recurrence risk is low. Studies of parental origin find the deleted chromosome is paternally derived in the large majority of these cases. |
| Unbalanced product of a parental balanced translocation | roughly 10% to 15% | A parent is healthy and has a normal amount of genetic material, just rearranged. At meiosis they can produce gametes missing 5p. Recurrence risk is substantial and the whole family needs studying. |
| Other | small minority | De novo unbalanced translocations, rings, mosaicism, and rare inversions. Mosaic cases can be considerably milder. |
The clinical trap. A balanced translocation carrier is phenotypically normal, has a normal chromosome count, and will never be found unless somebody looks. This is why identifying an unbalanced translocation in the child is not the end of the workup. It is the trigger for parental karyotypes. Getting this wrong means telling a family the recurrence risk is negligible when it is not.
Why the microscope is not enough
Conventional G-banded karyotyping is genuinely good at this. A classic cri du chat deletion is large, terminal, and on a chromosome whose banding pattern technologists know well. Many cases are called on the karyotype alone.
The problem is the tail of the distribution. Resolution on a routine metaphase spread is somewhere in the 5 to 10 megabase range, and it degrades fast on short, condensed chromosomes from a poor culture. Sitting under that limit are:
- small terminal deletions of a megabase or two, which is plenty of DNA to cause the syndrome and not nearly enough to move a band boundary visibly
- subtle unbalanced translocations, where 5p is replaced by material of similar size and staining from another chromosome, so the chromosome still looks the right length
- cases where the clinical suspicion is strong, the karyotype is normal, and somebody has to decide what to do next
There is also a practical constraint that has nothing to do with resolution: a karyotype needs dividing cells. Culture, harvest, spread, band, analyse. That is days. A neonate in a special care unit with an unexplained high-pitched cry is a question with a clock on it.
What iFISH adds
Interphase FISH removes the culture step entirely. A fluorescently labelled probe targeting 5p15.2 is hybridised to nuclei on a slide, alongside a control probe elsewhere on chromosome 5, commonly a centromeric or 5q31 target. Then you count signals.
- Two 5p15.2 signals in a nucleus means two intact copies of the region.
- One 5p15.2 signal against two control signals means one copy has been lost.
The control probe is doing real work here. Two signals from the control confirm that the cell is present, intact, and that hybridisation succeeded. A nucleus showing one red and one green is not a deletion, it is a failed hybridisation, and it must not be scored.
Because interphase nuclei do not need to divide, results come back in hours, from uncultured amniocytes, cord blood, buccal cells, or a direct preparation. Try it below.
[[ifish-scorer]]
Why cut-offs exist. If that drill produced a false positive, it was almost certainly a nucleus where two signals sat close enough together to read as one. That artefact happens in every laboratory, on every probe, which is why no result rests on a single nucleus. Laboratories establish a normal cut-off from control specimens, score a defined number of cells, typically 100 to 200 for a constitutional interphase assay, and require the abnormal fraction to exceed that cut-off before reporting. A distance criterion for calling two signals distinct, usually one signal diameter, is part of the same discipline.
Choosing the test
| Method | Resolution | Needs dividing cells | Best used when |
|---|---|---|---|
| G-banded karyotype | roughly 5 to 10 Mb | Yes | You need the whole-genome view. Only the karyotype shows you a balanced rearrangement, a marker chromosome, or the structure behind an unbalanced result. |
| Interphase FISH | targeted, well under 1 Mb | No | You have a specific question and a clock. Rapid confirmation, uncultured samples, mosaicism screening across many cells. |
| Metaphase FISH | targeted | Yes | You need to see where the missing material went, which interphase counting cannot tell you. |
| Chromosomal microarray | kilobase level | No | First-tier test for unexplained developmental delay. Sizes the deletion precisely and maps the breakpoints, which is what predicts severity. Cannot see balanced rearrangements. |
In practice these are complementary, not competing. A realistic 2026 workflow for a suspected case looks like this: microarray as the first-tier test because it sizes the deletion, karyotype in parallel or in follow-up because it is the only way to see the structure, interphase FISH when somebody needs an answer today, and parental karyotypes whenever the finding could be the unbalanced product of a translocation.
Writing it up
Your findings are only as good as the nomenclature you report them in. These are the strings you should be able to produce without looking them up.
| Finding | ISCN |
|---|---|
| Terminal 5p deletion, female, short form | 46,XX,del(5)(p15.2) |
| The same, detailed form | 46,XX,del(5)(:p15.2→qter) |
| Interstitial deletion, male | 46,XY,del(5)(p14.3p15.2) |
| Interstitial, detailed form | 46,XY,del(5)(pter→p15.2::p14.3→qter) |
| Unbalanced product of a maternal translocation | 46,XX,der(5)t(5;7)(p15.2;q34)mat |
| The healthy carrier parent | 46,XX,t(5;7)(p15.2;q34) |
| Interphase FISH, deletion present | nuc ish(D5S23,D5S721)x1,(D5Z1)x2[200] |
| Microarray, illustrative coordinates | arr[GRCh38] 5p15.33p15.2(113,576_11,700,000)x1 |
Probe identifiers must match the probe set your laboratory actually validated, and the array coordinates above are illustrative rather than from a specific case. The grammar is what transfers.
Ninety seconds, if you would rather watch it
The same story, compressed.
[[cdc-short]]
Five things to walk away with
| # | Takeaway | Why it matters at the bench |
|---|---|---|
| 1 | 5p15.3 gives the cry, 5p15.2 takes the development | Two adjacent bands, two separable phenotypes. Deletion extent, not the diagnosis label, predicts outcome. |
| 2 | Deletion size spans several orders of magnitude | From a few hundred kilobases to over 40 Mb. This is why microarray earns its place: it sizes what the karyotype only names. |
| 3 | Always ask where the deletion came from | Roughly 10% to 15% are unbalanced products of a healthy parent's balanced translocation. Missing that misstates the recurrence risk. |
| 4 | A normal karyotype does not exclude the diagnosis | Small terminal deletions and cryptic unbalanced translocations live under the resolution limit. Escalate to FISH or microarray. |
| 5 | Two signals is normal, one signal is your answer, and the control probe decides whether you may score at all | One red with one green is a failed hybridisation, not a deletion. Respect the cut-off and the cell count. |
Lejeune heard something in a nursery and went back to look at chromosomes. Sixty-three years later the resolution has improved by four orders of magnitude, the cells no longer have to divide, and the answer can come back the same afternoon. What has not changed is that somebody has to sit down, look carefully, apply a criterion, and decide what the signals mean. That is still the job.
References and further reading
- Lejeune J, Lafourcade J, Berger R, et al. Trois cas de délétion partielle du bras court d'un chromosome 5. C R Hebd Seances Acad Sci. 1963;257:3098-3102.
- Overhauser J, Huang X, Gersh M, et al. Molecular and phenotypic mapping of the short arm of chromosome 5: sublocalization of the critical region for the cri-du-chat syndrome. Hum Mol Genet. 1994;3(2):247-252.
- Mainardi PC. Cri du chat syndrome. Orphanet J Rare Dis. 2006;1:33.
- Nguyen JM, Qualmann KJ, Okashah R, et al. 5p deletions: current knowledge and future directions. Am J Med Genet C Semin Med Genet. 2015;169(3):224-238.
- McGowan-Jordan J, Hastings RJ, Moore S, eds. ISCN 2020: An International System for Human Cytogenomic Nomenclature. Karger.
- Miller DT, Adam MP, Aradhya S, et al. Consensus statement: chromosomal microarray is a first-tier clinical diagnostic test for individuals with developmental disabilities or congenital anomalies. Am J Hum Genet. 2010;86(5):749-764.
This article is educational and is not clinical guidance. Diagnostic and reporting practice follows your own laboratory's validated procedures and current professional standards.