ISCN Notation Examples: How to Read and Write a Karyotype
Last updated: October 22, 2026
ISCN, the International System for Human Cytogenomic Nomenclature, is the standard way to write a chromosome result. This page explains how an ISCN karyotype is built and works through the examples you will meet most often, from 46,XX to cancer karyotypes with several clones.
What ISCN is
ISCN is maintained by an international standing committee and revised every few years. It covers karyotypes from banded chromosomes and also results from FISH, chromosomal microarray and sequencing. This page focuses on karyotype notation, which is what you write after G-banded analysis. Always check the current edition of ISCN for the complete rules.
The basic structure of a karyotype
Every karyotype is written in the same order, with commas between the parts and no spaces:
- the total number of chromosomes;
- the sex chromosomes;
- any abnormalities. Sex chromosome abnormalities come first, then autosomes in numerical order. For any one chromosome, a numerical change is listed before a structural one.
So a normal female is 46,XX and a normal male is 46,XY.
Key symbols and abbreviations
| Symbol | Meaning |
|---|---|
| + / − | Gain or loss of a whole chromosome, written before the chromosome: +21, −7 |
| p, q | Short arm and long arm |
| ( ) | Enclose the chromosomes and the breakpoints of a structural change |
| ; | Separates chromosomes and breakpoints in a change that involves more than one chromosome |
| [ ] | The number of cells in a cell line or clone |
| / | Separates cell lines (mosaicism) or clones |
| mos, chi | Mosaic (cell lines from one zygote) and chimera (cell lines from different zygotes) |
| del, dup | Deletion and duplication |
| inv | Inversion |
| t | Reciprocal translocation |
| rob | Robertsonian translocation |
| der | Derivative chromosome: a chromosome produced by a rearrangement |
| i | Isochromosome |
| r | Ring chromosome |
| add | Additional material of unknown origin |
| mar | Marker chromosome: a structurally abnormal chromosome that banding cannot identify |
| ? | Questionable identification of a chromosome or structure |
Band names are read digit by digit. q11.2 is "q one one point two": long arm, region 1, band 1, sub-band 2.
Numerical abnormalities
| Karyotype | Meaning |
|---|---|
| 47,XX,+21 | Female with trisomy 21 (Down syndrome). Worked case |
| 47,XY,+18 | Male with trisomy 18 (Edwards syndrome). Worked case |
| 47,XX,+13 | Female with trisomy 13 (Patau syndrome). Worked case |
| 45,X | A single X chromosome (Turner syndrome). Written 45,X, not 45,X,−X. Worked case |
| 47,XXY | Male with an extra X (Klinefelter syndrome). Worked case |
| 47,XXX | Female with an extra X (trisomy X) |
| 47,XYY | Male with an extra Y |
| 69,XXY | Triploidy: three copies of every chromosome |
Sex chromosome gains and losses are shown in the sex chromosome part of the karyotype, so there is no + sign in 47,XXY. Autosomal gains and losses always carry a + or − sign.
Mosaicism
When a person has more than one cell line, each line is written in full, followed by the number of cells in square brackets, with the lines separated by a slash. In a constitutional karyotype the normal cell line, when present, is listed last.
| Karyotype | Meaning |
|---|---|
| mos 47,XX,+21[8]/46,XX[22] | Mosaic trisomy 21: 8 cells with trisomy 21 and 22 normal cells |
| mos 45,X[12]/46,XX[18] | Mosaic Turner syndrome: 12 cells with a single X and 18 normal female cells |
Structural abnormalities
A structural change is written as the abbreviation, then the chromosome or chromosomes in parentheses, then the breakpoints in a second set of parentheses.
| Karyotype | Meaning |
|---|---|
| 46,XX,del(5)(p15.2) | Terminal deletion of the short arm of chromosome 5 from band p15.2 to the end, as seen in cri du chat syndrome. A single breakpoint means the deletion runs to the end of the arm. |
| 46,XY,del(15)(q11.2q13) | Interstitial deletion within 15q, between bands q11.2 and q13, the region involved in Prader-Willi and Angelman syndromes. Two breakpoints mean an interstitial deletion. |
| 46,XX,t(11;22)(q23;q11.2) | Balanced reciprocal translocation between 11q23 and 22q11.2. The breakpoints are listed in the same order as the chromosomes. |
| 47,XY,+der(22)t(11;22)(q23;q11.2) | An extra derivative chromosome 22 from that translocation, the karyotype of Emanuel syndrome, usually inherited from a parent who carries the balanced form |
| 45,XX,rob(13;14)(q10;q10) | Balanced carrier of a Robertsonian translocation between 13 and 14, the most common Robertsonian. Two acrocentrics are joined into one chromosome, so the count is 45. q10 marks the centromere. |
| 46,XY,rob(14;21)(q10;q10),+21 | Translocation Down syndrome. The count is 46 because the Robertsonian chromosome counts as one, yet there are three copies of 21q. |
| 46,XX,inv(9)(p12q13) | Pericentric inversion of chromosome 9, a common variant generally considered benign |
| 46,X,i(X)(q10) | An isochromosome made of two copies of the long arm of X, in place of one normal X. A structural form of Turner syndrome. |
| 46,XX,r(7)(p22q36) | Ring chromosome 7, formed by breaks near both ends at p22 and q36 |
| 46,XX,add(7)(q32) | Material of unknown origin attached to 7q at band q32 |
| 47,XY,+mar | An extra marker chromosome whose origin cannot be identified by banding |
Cancer karyotypes
Cancer karyotypes describe clones rather than constitutional cell lines, and they do not use mos. Every clone carries its cell count in square brackets, and the normal cells, if any, are listed last.
| Karyotype | Meaning |
|---|---|
| 46,XY,t(9;22)(q34;q11.2)[20] | The Philadelphia chromosome translocation in all 20 cells analyzed, typical of chronic myeloid leukemia. Worked case |
| 46,XX,t(15;17)(q24;q21)[18]/46,XX[2] | The t(15;17) of acute promyelocytic leukemia in 18 cells, with 2 normal cells |
| 47,XY,+8[6]/46,XY[14] | A clone with trisomy 8 in 6 cells, with 14 normal cells |
| 45,XY,−7[10]/46,XY[10] | A clone with monosomy 7 in 10 cells, with 10 normal cells |
| 46,XX,del(5)(q13q33)[15]/46,XX[5] | An interstitial deletion of 5q in 15 cells, a recurrent change in myelodysplastic syndromes |
What counts as a clone: at least two cells with the same gain or the same structural rearrangement, or at least three cells with the same loss. A single abnormal cell is not reported as a clone.
FISH and microarray results
ISCN also defines notation for other methods. FISH on metaphase chromosomes is written with ish, FISH on interphase nuclei with nuc ish, and chromosomal microarray results with arr. A microarray result gives the genome build, the band, the genomic coordinates of the change and its copy number, for example x1 for a deletion and x3 for a duplication. When a karyotype and FISH are both done, the results are joined with a full stop.
Common ISCN mistakes
- Adding spaces. A karyotype has none, except after mos or chi.
- Listing abnormalities out of order. Sex chromosomes first, then autosomes from lowest to highest number.
- Mixing up separators. A comma separates parts of the karyotype, a semicolon separates chromosomes inside a rearrangement, and a slash separates cell lines.
- Miscounting after a Robertsonian translocation. The joined chromosome counts as one.
- Writing band names as numbers. q11.2 is not "q eleven point two", and q112 is not a band.
- Forgetting cell counts in mosaic and cancer karyotypes.
Keep learning
This page is for education and training. It is not medical advice and must not be used to diagnose or treat anyone. Laboratory results should be interpreted by qualified professionals following current standards.