ISCN Nomenclature Cheat Sheet for the ASCP CG Exam

Published 2026-04-16

The International System for Human Cytogenomic Nomenclature (ISCN) is the language cytogenetics speaks. On the ASCP CG exam, you'll be asked to write, read, and interpret ISCN strings — sometimes from a karyogram image, sometimes from a clinical scenario. This cheat sheet covers the high-yield rules.

The full standard is published by Karger as ISCN 2024. It's worth owning a copy if you'll be working in the field.

Test your ISCN fluency: Take our free Initial Assessment →

The Standard Karyotype String Format

[Total chromosome count],[Sex chromosomes],[Abnormalities separated by commas]

Examples:

• 46,XX — normal female
• 46,XY — normal male
• 47,XX,+21 — Down syndrome (female)
• 45,X — Turner syndrome
• 47,XXY — Klinefelter syndrome

Essential Abbreviations

Reading Band Locations

Format: chromosome[arm][region][band].[sub-band]

Example: 7q31.2 = chromosome 7, long arm, region 3, band 1, sub-band 2.

Numerical Abnormalities

• Gains/losses use + or − BEFORE the chromosome.
• 47,XY,+21 — extra chromosome 21
• 45,XX,−22 — loss of chromosome 22
• 48,XXY,+21 — Klinefelter + Down syndrome

Structural Abnormalities — Syntax to Memorize

Translocations: t(chromosomes)(breakpoints)

46,XY,t(9;22)(q34;q11.2)

Reciprocal translocation between chromosome 9 (long arm, band 34) and chromosome 22 (long arm, band 11.2). This is the Philadelphia chromosome — diagnostic for CML.

Deletions: del(chromosome)(breakpoints)

• Terminal deletion: 46,XX,del(5)(p14) — deletion from 5p14 to terminus (Cri-du-chat)
• Interstitial deletion: 46,XY,del(15)(q11.2q13) — deletion between two breakpoints (Prader-Willi/Angelman region)

Inversions: inv(chromosome)(breakpoints)

• Paracentric (both breaks on same arm): 46,XY,inv(7)(q22q34)
• Pericentric (breaks on both arms — spans centromere): 46,XX,inv(9)(p12q13) — common benign variant

Duplications, Isochromosomes, Rings

• 46,XX,dup(1)(q21q32) — duplication of segment
• 46,X,i(X)(q10) — isochromosome of Xq (common in Turner mosaics)
• 46,XX,r(13)(p13q34) — ring chromosome 13

Derivative Chromosomes

der(chromosome) describes a structurally rearranged chromosome with a known origin. Used in unbalanced translocations:

46,XY,der(14;21)(q10;q10),+21

Robertsonian translocation Down syndrome.

Mosaicism

Use mos and separate clones with a slash, listing the largest clone first:

mos 47,XXY[18]/46,XY[7]

18 cells with XXY, 7 cells with normal male karyotype.

Common Exam Traps

1. Order matters within an abnormality: sex chromosomes before autosomes; lower-numbered chromosomes before higher.
2. Brackets [ ] indicate cell counts in mosaic and tumor karyotypes.
3. + placement: before a whole chromosome, but after a structural abnormality (e.g., +der(...)).
4. p arm bands count outward from centromere — p11 is closer to the centromere than p15.
5. A translocation is "balanced" only if no genetic material is gained or lost — t(9;22) is balanced; der(9)t(9;22) alone is not.

Worked Examples

A Few Authoritative References

• ISCN 2024 (Karger) — the official standard
• ACMG Technical Standards — clinical interpretation guidance
• NCBI Genetics Bookshelf — free chromosomal nomenclature primer

Practice Reading Real Karyotypes

Take the free 20-question Initial Assessment → — includes ISCN-interpretation questions weighted like the real ASCP CG exam.

Want hundreds more ISCN drills, image-based karyotyping practice, and full adaptive mock exams? Explore CruxSci membership →

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