Fluorescent in situ hybridization, commonly called FISH, is a laboratory technique that uses glowing DNA probes to locate specific genes or chromosome regions inside cells. It works by attaching a small, fluorescent-tagged piece of DNA to its matching sequence on a chromosome, then viewing the result under a specialized microscope. The technique can reveal whether genes are missing, duplicated, or rearranged, making it one of the most widely used tools in cancer diagnosis and prenatal screening.
How FISH Works at the Molecular Level
The process starts with a biological sample, which could be blood, bone marrow, amniotic fluid, or a slice of preserved tissue from a biopsy. Cells from the sample are spread onto a glass slide and treated so the chromosomes inside become accessible. The double-helix structure of the DNA is then unwound, or “denatured,” separating the two strands so a probe can reach in and bind.
The probe is the key ingredient. It’s a short, purified stretch of DNA designed to match a specific gene or chromosome region, and it’s tagged with a fluorescent dye. When the probe is washed over the slide, it seeks out and locks onto its complementary sequence. After rinsing away any unbound probe, a technician examines the slide under a fluorescence microscope. Wherever the probe has attached, a bright spot of color appears, pinpointing exactly where that gene sits on the chromosome and, critically, how many copies are present.
Results typically come back within a few days, which is significantly faster than conventional cytogenetic testing, where cells must be cultured and can take weeks.
Types of Probes Used in FISH
Different probes target different parts of a chromosome, and the choice depends on what the lab is looking for:
- Centromere probes bind to the central region of a chromosome and are used to count how many copies of that chromosome a cell contains.
- Gene-specific (locus-specific) probes target a particular gene to detect deletions, duplications, or amplifications at that exact location.
- Probe panels combine several differently colored locus-specific probes in a single mix, allowing technicians to evaluate multiple targets at once.
When two probes labeled with different colors overlap and produce a combined signal (often yellow from a red and green pair), it indicates that two genes have fused together. This is how FISH detects the chromosomal rearrangements that drive many cancers.
FISH in Cancer Diagnosis and Treatment
FISH has become essential in oncology because many cancers are defined not just by where they grow, but by specific genetic changes inside the tumor cells. Identifying those changes determines which treatments will work.
In breast cancer, FISH measures whether the HER2 gene has been amplified, meaning the cell has made too many copies of it. Tumors with HER2 amplification grow more aggressively but respond to targeted therapies that block HER2 signaling. Under current guidelines from the American Society of Clinical Oncology and the College of American Pathologists, FISH is one of the standard methods for classifying HER2 status, and the result directly shapes the treatment plan. Even tumors without full amplification may carry low levels of HER2 protein that make patients eligible for newer drug-delivery therapies, so precise measurement matters.
In lung cancer, FISH detects rearrangements in the ALK and ROS1 genes using “break-apart” probes. These probes are designed so that if the gene is intact, two colored signals sit close together. If the gene has broken and rearranged with a partner, the signals split apart. A positive result opens the door to targeted inhibitors that are often more effective and better tolerated than traditional chemotherapy.
The technique is also used in brain tumors (gliomas), where a co-deletion of regions on chromosomes 1 and 19 predicts a better response to treatment; in Ewing sarcoma, where an EWSR1 rearrangement confirms the diagnosis; and in rare soft-tissue tumors like dermatofibrosarcoma, where a specific gene fusion makes the tumor sensitive to a targeted drug.
FISH in Prenatal and Newborn Screening
Outside of cancer, FISH is most commonly used to screen for chromosomal abnormalities before or shortly after birth. Up to 95% of chromosomal problems diagnosed prenatally involve extra or missing copies of just five chromosomes: 13, 18, 21, X, and Y. FISH can check all five in a single test.
The conditions this covers are some of the most well-known genetic disorders. An extra copy of chromosome 21 causes Down syndrome, an extra chromosome 18 causes Edwards syndrome, and an extra chromosome 13 causes Patau syndrome. Abnormal numbers of sex chromosomes produce conditions like Turner syndrome (a single X in females) or Klinefelter syndrome (XXY in males). In newborns, abnormalities of these five chromosomes account for roughly 81% to 95% of all major chromosomal anomalies, which is why FISH focusing on this limited set captures the vast majority of clinically significant findings.
Because FISH does not require cells to be grown in culture, it delivers answers much faster than a full karyotype, which is especially valuable when families are waiting for results during pregnancy.
What Samples Can Be Tested
FISH is flexible in the types of biological material it can analyze. Common sample sources include bone marrow aspirates (for blood cancers), blood draws, amniotic fluid or placental tissue (for prenatal screening), touch preparations from lymph nodes, and formalin-fixed, paraffin-embedded tissue blocks from surgical biopsies. That last category is particularly useful because it means FISH can be performed on archived tissue, allowing doctors to go back and test a stored biopsy for a new genetic marker if treatment decisions change.
Limitations and Sources of Error
FISH is highly targeted, which is both its strength and its weakness. Each probe only looks for the specific sequence it was designed to find. If a clinically important abnormality sits somewhere else in the genome, FISH will miss it entirely. Broader techniques like chromosomal microarray or whole-genome sequencing cast a wider net, though they come with their own tradeoffs in cost and turnaround time.
Sample quality plays a major role in accuracy. Over-fixation of tissue creates excessive chemical cross-links between proteins, trapping the DNA so the probe cannot reach its target. The result is weak or absent fluorescent signals that can be misread as a deletion when the gene is actually there. Conversely, degraded or aging tissue can produce the opposite problem. Studies comparing old and recent slides of the same tumor type have shown that signal intensity drops as tissue blocks age, with blocks over five years old showing notably weaker HER2 signals. Old slides can also generate false positive results due to background fluorescence noise from denatured proteins.
These issues mean that labs follow strict protocols for tissue handling and set minimum thresholds for how many cells must show an abnormal signal before a result is called positive. For gene fusions, for example, the abnormality generally must be present in at least 10% of analyzed cells.
Multiplex FISH and Advanced Variations
Standard FISH uses one or a handful of probes at a time. Multiplex FISH, or M-FISH, pushes the technology further by labeling all 24 human chromosome types (22 pairs plus X and Y) with distinct color combinations, producing a full-color karyotype in a single experiment. This makes it possible to identify complex rearrangements that conventional methods would struggle to untangle.
M-FISH has proven especially valuable in two scenarios in cancer genetics: tumors with apparently normal chromosomes that may be hiding small rearrangements invisible to standard analysis, and tumors with massively rearranged genomes where the sheer number of abnormalities makes traditional karyotyping impractical. Because M-FISH can accurately analyze individual cells without pre-selecting which chromosomes to examine, it is also used in chromosome breakage studies and to track how tumors evolve genetically over time.