FISH Probe Types Explained: Locus-Specific, Centromeric, Break-Apart, Dual-Fusion
Published 2026-04-17
Fluorescence In Situ Hybridization (FISH) is one of the highest-yield topics on the ASCP CG exam. You'll be expected to identify the right probe for the right clinical question and interpret signal patterns in both metaphase and interphase nuclei. Here's the cheat sheet.
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The Four Major FISH Probe Types
1. Locus-Specific Identifier (LSI) Probes
LSI probes hybridize to a unique DNA sequence at a specific chromosomal locus. They're used to detect deletions, amplifications, or copy number changes of a known gene region.
- Normal signal pattern: 2 signals (one per homolog)
- Clinical use: Detecting TP53 deletion in CLL, MYCN amplification in neuroblastoma, EGFR in glioblastoma.
- Reference: NCBI overview of FISH probe types
2. Centromere Enumeration Probes (CEP / α-satellite)
CEP probes target the highly repetitive alpha-satellite DNA in centromeres. Each chromosome has its own centromere-specific repeat sequence, so probes can be made specific to one chromosome.
- Normal signal pattern: 2 signals
- Clinical use: Aneuploidy detection — trisomy 13, 18, 21, X, Y. Often used in prenatal interphase FISH panels (AneuVysion).
- Caveat: Acrocentric chromosomes (13, 14, 15, 21, 22) and chromosomes 13/21 share similar α-satellite sequences, so dedicated probes use other repeats.
3. Whole Chromosome Paint (WCP) Probes
A "cocktail" of probes covering the entire length of one chromosome, fluorescently labeling it end-to-end.
- Normal signal pattern: Two fully painted homologs
- Clinical use: Identifying the origin of marker chromosomes and characterizing complex translocations. Best used on metaphase spreads (not interphase).
- Limitation: Cannot resolve small intra-chromosomal rearrangements.
4. Break-Apart and Dual-Fusion Probes
These are the two strategies for translocation detection — and ASCP loves to test the difference.
Break-Apart Probes
Two differently colored probes flank a known breakpoint region within a single gene.
- Normal signal pattern: 2 fused (yellow) signals — one per homolog
- Abnormal pattern: 1 fused + 1 red + 1 green (split signal = rearrangement of that gene with any partner)
- Clinical use: MLL (KMT2A) rearrangements in AML, ALK in lung cancer, MYC in lymphoma.
- Why use it: When you care that a gene is rearranged but don't know (or care about) the partner.
Dual-Fusion Probes
Two probes target two different genes on two different chromosomes — one labeled red, one green.
- Normal signal pattern: 2 red + 2 green (separate)
- Abnormal pattern: 1 red + 1 green + 2 fused (yellow) signals — confirming the specific translocation
- Clinical use: BCR-ABL1 t(9;22) in CML, PML-RARA t(15;17) in APL, IGH-MYC t(8;14) in Burkitt lymphoma.
- Why use it: When you need to confirm a specific fusion partner.
Quick Comparison Table
| Probe Type | Normal Signal | Detects | Best On |
|---|---|---|---|
| LSI | 2 signals | Deletions, amplifications | Interphase + metaphase |
| CEP | 2 signals | Aneuploidy | Interphase |
| WCP | 2 painted homologs | Marker ID, complex rearrangements | Metaphase only |
| Break-apart | 2 fused | Gene rearrangement (any partner) | Interphase + metaphase |
| Dual-fusion | 2R + 2G separate | Specific translocation | Interphase + metaphase |
High-Yield Clinical Pairings to Memorize
| Translocation | Disease | Probe Strategy |
|---|---|---|
| t(9;22) BCR-ABL1 | CML, ALL | Dual-fusion |
| t(15;17) PML-RARA | APL | Dual-fusion |
| t(8;14) IGH-MYC | Burkitt lymphoma | Dual-fusion |
| t(11q23) MLL/KMT2A | AML, ALL | Break-apart |
| t(14;18) IGH-BCL2 | Follicular lymphoma | Dual-fusion |
| del(17p) TP53 | CLL | LSI |
| +12 trisomy | CLL | CEP 12 |
Signal Scoring Tips for the Exam
- Always score at least 200 nuclei for interphase FISH.
- Establish your lab's normal cutoff using known-negative controls (typically mean + 3 SD).
- Watch for technical pitfalls: split signals from incomplete hybridization can mimic a true rearrangement.
- For more detail on official guidelines, see the American College of Medical Genetics (ACMG) technical standards.
Putting It Together
If the exam asks "which probe would you use to detect…", ask yourself:
- Am I counting chromosomes? → CEP
- Am I looking for a deletion/amplification of a known gene? → LSI
- Am I confirming a specific translocation partner? → Dual-fusion
- Do I just need to know if a gene is broken? → Break-apart
- Do I need to ID an unknown marker chromosome? → WCP (metaphase)
Ready to Test Yourself?
Take the free 20-question Initial Assessment → — includes molecular cytogenetics questions weighted to mirror the real ASCP CG exam.
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