A FISH analysis, short for fluorescence in situ hybridization, is a laboratory technique that uses glowing molecular markers to find specific DNA sequences or chromosomal changes inside your cells. It works by attaching fluorescent-tagged pieces of DNA (called probes) to matching sequences on your chromosomes, then viewing the results under a special microscope. Doctors use it to diagnose genetic conditions, detect cancer-related changes, and screen for chromosomal abnormalities during pregnancy.
How the Test Works
Think of FISH like highlighting specific passages in a book with glow-in-the-dark ink. Your DNA is made of two strands twisted together, and those strands are held together because their sequences match, like teeth on a zipper. In a FISH test, lab technicians first separate those two strands in both the sample and the probe. When the single-stranded probe is added to the sample, it finds and locks onto the matching DNA sequence. This binding step is called hybridization.
Once the probes have attached, technicians wash away anything that didn’t bind and examine the sample under a fluorescence microscope. The probes light up in specific colors, revealing whether a particular gene or chromosome region is present, missing, duplicated, or rearranged. Most samples sit overnight for hybridization, but when results are urgent, labs can complete the process in about four hours.
What the Signals Mean
The glowing dots (signals) that appear under the microscope follow recognizable patterns depending on the type of probe used. The two most common probe designs are break-apart probes and fusion probes, and each tells a different story.
Break-apart probes use two colors, typically red and green, that sit right next to each other on the same gene. In a normal cell, the red and green signals overlap and appear as a single yellow dot. If the gene has been rearranged or broken, you see a separated red signal and a separated green signal instead. Multiple yellow signals without any separated colors is considered a negative (normal) result. If one color disappears entirely while the other remains, that points to a deletion, where part of the gene has been lost.
Fusion probes work in the opposite direction. They’re designed to light up when two genes that are normally on different chromosomes have been pushed together by a translocation. Each gene gets its own color, and a new combined signal appears only if those genes have fused. Fusion probes are less commonly available commercially, but they’re useful for confirming specific gene fusions when break-apart results are unclear.
FISH vs. Traditional Karyotyping
Before FISH became widely available, the standard method for examining chromosomes was karyotyping, where cells are grown in the lab, stained, and photographed so a technician can visually inspect each chromosome. Karyotyping can only detect changes larger than about 5 million base pairs (5 Mb) and requires live, actively dividing cells. Preparation alone can take several days.
FISH has a much finer resolution, detecting changes as small as 100,000 to 1 million base pairs. That’s roughly 5 to 50 times more precise. It also doesn’t require cells to be dividing, which makes it faster and usable on a wider range of samples. The tradeoff is that FISH only tells you about the specific gene or region you’re testing for. Karyotyping gives a broad view of all 46 chromosomes at once, while FISH is a targeted search for something specific.
Prenatal Screening
One of the most common uses of FISH is in prenatal testing, where speed matters. When amniocentesis or chorionic villus sampling collects fetal cells, FISH can screen for the most clinically significant chromosomal abnormalities within 24 to 48 hours, compared to the 10 to 14 days a full karyotype typically requires.
Prenatal FISH panels usually target five chromosomes: 13, 18, 21, X, and Y. Abnormalities involving these five chromosomes account for roughly 95% of chromosomal conditions that cause birth defects. Trisomy 21 (Down syndrome) is the most frequently detected, followed by monosomy X (Turner syndrome) and trisomy 13 (Patau syndrome). The rapid turnaround can significantly reduce the waiting period for expectant parents.
Cancer Diagnosis and Treatment
FISH plays a critical role in cancer care because many cancers are driven by specific chromosomal rearrangements, and identifying those rearrangements often determines which treatment will work best.
Breast Cancer and HER2 Status
In breast cancer, FISH is used to determine whether the HER2 gene is amplified, meaning the cell has too many copies of it. HER2-positive tumors grow more aggressively but respond to targeted therapies that block the HER2 protein. A tumor is considered HER2-amplified when the ratio of HER2 signals to a reference signal is 2 or greater, or when the average HER2 copy number per cell is 6 or more. This distinction directly determines whether a patient is eligible for HER2-targeted treatment. The American Society of Clinical Oncology and the College of American Pathologists recommend this testing as standard practice.
Blood Cancers
In leukemia, lymphoma, and multiple myeloma, FISH panels test for a range of chromosomal translocations, where pieces of two chromosomes swap places and create abnormal fusion genes. One well-known example is the translocation between chromosomes 9 and 22, which creates a fusion gene that drives chronic myeloid leukemia. FISH panels for blood cancers routinely test for a dozen or more such rearrangements, and the specific pattern found helps classify the cancer’s risk level and guide treatment decisions. Recent 2025 guidelines for multiple myeloma specifically recommend using FISH results to classify patients into standard-risk and high-risk categories.
What FISH Cannot Detect
The main limitation of FISH is that it only provides information about the specific gene or region being tested. If the probe isn’t designed to look at a particular spot on the genome, any abnormality there will be invisible. Small-scale changes like single-letter mutations in the DNA code, which are the kind of changes found in conditions like sickle cell disease or cystic fibrosis, are too small for FISH to detect. Those require different molecular techniques like DNA sequencing.
FISH also can’t detect structural changes that don’t alter the probe’s binding site. If a chromosome has a small inversion, where a segment flips around but stays in the same general location, the probe may still bind normally and the rearrangement goes unnoticed. For this reason, FISH is almost always used alongside other genetic tests rather than as a standalone diagnostic tool. It excels at answering a specific, targeted question quickly and reliably, but it’s not a comprehensive scan of your entire genome.