What Does FISH Amplification Mean?

FISH stands for fluorescence in situ hybridization. It is a laboratory test that lights up one specific piece of DNA inside a cell so a pathologist can count it.

Amplification means the cell has extra copies of that piece. Not a spelling change in the gene. Not a gene fused to another gene. Just more of it than there should be.

A FISH report almost never says only "amplified." It carries numbers. Those numbers, not the word, are what drives the treatment decision.

The National Human Genome Research Institute describes the method plainly. The lab builds short single-stranded pieces of DNA that match part of the gene in question. These are called probes. Each probe is tagged with a fluorescent dye.

DNA strands pair with their matching sequence. So the probe sticks to its target on the chromosome, and the dye marks the spot. Under the microscope, each copy of the gene appears as a colored dot inside a cell nucleus.

NHGRI describes three probe types. Locus-specific probes target one region and are the kind used to count gene copies. Alphoid or centromeric repeat probes bind the repeating DNA at the center of a chromosome and are used to check how many copies of the whole chromosome are present. Whole-chromosome probes paint an entire chromosome in one color, which is how a lab spots a piece of one chromosome stuck onto another.

Amplification testing uses the first two together. One color counts the gene. A second color counts the chromosome it sits on. That second count is the reason the report has a ratio.

If a cell gains an extra copy of an entire chromosome, every gene on that chromosome goes up. That is not the same biological event as one gene being copied over and over.

The ratio separates the two. It divides the gene count by the control count. A high count with a normal ratio suggests the chromosome multiplied. A high count with a high ratio suggests the gene itself was selectively amplified.

HER2 testing is where most patients meet this word. HER2 is a gene whose protein drives growth in some breast and stomach cancers, and it is the target of several drugs.

The dual-probe test counts HER2 signals and CEP17 signals in the same cells. CEP17 is the centromere of chromosome 17, which is where HER2 lives. The pathologist reports two numbers: the average HER2 copies per cell, and the HER2 to CEP17 ratio.

The ASCO and College of American Pathologists guideline sorts every result into one of five groups.

• Group 1: ratio 2.0 or higher, and average HER2 copies 4.0 or higher. Reported as ISH positive.
• Group 2: ratio 2.0 or higher, but average HER2 copies below 4.0.
• Group 3: ratio below 2.0, but average HER2 copies 6.0 or higher.
• Group 4: ratio below 2.0, and average HER2 copies 4.0 or higher but below 6.0.
• Group 5: ratio below 2.0, and average HER2 copies below 4.0. Reported as ISH negative.

ISH means in situ hybridization, the family of tests FISH belongs to.

Groups 2, 3, and 4 are the borderline zone. The guideline does not let the lab call them on FISH numbers alone.

For those three groups, the pathologist must also review immunohistochemistry, or IHC, on the same specimen. IHC is a stain that measures how much HER2 protein sits on the cell surface, scored 0, 1+, 2+, or 3+. In Groups 3 and 4, the algorithm turns on that stain: an IHC score of 3+ gives a positive final result, an IHC of 0 or 1+ gives a negative one with a comment, and a 2+ sends the slide back for a blinded recount before the case is settled.

This is why a report can read "HER2 amplified by FISH" in one line and "HER2 negative" as the final result. Both can be true. The word describes the raw count. The final line reflects the full algorithm.

If your report shows Group 2, 3, or 4, the question worth asking is simple: what was the IHC score, and what did the final combined result come out as?

Amplification does not mean "give a targeted drug" everywhere. In neuroblastoma, a childhood cancer of nerve tissue, MYCN amplification is mainly a risk marker.

The NCI PDQ summary treats MYCN status as foundational to risk group assignment. The consequences are concrete. Infants younger than 12 months with INSS stage 4 disease and MYCN amplification are placed in the high-risk group. PDQ reports 5-year event-free survival of 37% and overall survival of 45% for that group. Stage 2B patients with MYCN amplification also do worse than those without it.

PDQ also notes a practical testing detail: MYCN amplification can be tested on involved bone marrow when at least 30% of the marrow contains tumor.

So the same finding can select a drug in one cancer and select an intensity of therapy in another.

NHGRI notes that microarray technology has replaced FISH for many uses. FISH survives in oncology because it does something sequencing panels do less cleanly. It counts copies cell by cell, in tissue, with the architecture intact, so the pathologist can confirm the counted cells are actually tumor.

That matters when a biopsy is small or mixed with normal tissue. NCI lists insufficient tumor tissue as one of the reasons biomarker testing fails to help. NCI also lists other failure points: tissue that cannot be safely obtained, no biomarker that matches an available drug, a matched drug that insurance will not cover, and a matched drug available only through a trial you cannot reach.

• What were the two numbers, the average copies per cell and the ratio?
• Which ISH group did the lab assign, and what was the IHC score?
• What is the final combined result, not the FISH line alone?
• How many cells were counted, and was the tumor content adequate?
• Does this result open a specific drug, change risk group, or neither?
• Was this run on the original biopsy or on a newer sample?

• https://www.genome.gov/about-genomics/fact-sheets/Fluorescence-In-Situ-Hybridization
• https://documents.cap.org/documents/her2_breast_update_algorithms_2023.pdf
• https://www.cancer.gov/types/neuroblastoma/hp/neuroblastoma-treatment-pdq
• https://www.cancer.gov/about-cancer/treatment/types/biomarker-testing-cancer-treatment