Exome testing, also called whole exome sequencing (WES), is a genetic test that reads the protein-coding portions of your DNA to look for mutations that cause disease. These coding regions make up only about 1.5% of your total genome, but they contain the instructions for building every protein your body needs. Because most known disease-causing mutations sit within this small slice of DNA, exome testing can scan roughly 20,000 genes at once without needing to sequence the other 98.5%.
What Exome Testing Actually Looks At
Your DNA is enormous: about 3 billion individual letters of genetic code. The vast majority of that sequence doesn’t directly code for proteins. The exome is the collective name for the parts that do. Think of your full genome as an entire library and the exome as just the sentences that contain actionable instructions.
During exome testing, a lab isolates those coding regions from a blood or tissue sample, then reads them using high-speed sequencing technology. The resulting data is compared against a reference human genome to spot differences, called variants. Software filters and ranks those variants by how likely they are to cause disease, drawing on databases of known mutations, population frequency data, and clinical literature. A team of geneticists and molecular pathologists then reviews the flagged variants to determine which, if any, explain the patient’s symptoms.
Who Gets This Test and Why
Exome testing is most commonly ordered for people with suspected genetic conditions that haven’t been diagnosed through standard workups or smaller gene panels. This includes children with developmental delays, neurological problems, or multiple birth defects, as well as adults with unexplained muscle, heart, or metabolic diseases. It’s particularly useful when a patient’s symptoms don’t point to one specific condition, because the test casts a wide net across all protein-coding genes rather than checking a predetermined list.
The overall diagnostic rate for exome sequencing in adults with rare disease is around 16%, based on a large retrospective study of over 2,000 patients. That number climbs significantly when both biological parents are tested alongside the patient, an approach called trio testing.
Trio Testing vs. Testing Alone
One of the biggest factors in whether exome testing delivers an answer is whether your biological parents can also provide samples. When three people are sequenced together (the patient plus both parents), the lab can immediately see which variants are inherited from which parent and which ones are brand new, appearing for the first time in the patient. These new, or “de novo,” mutations are often the culprits behind genetic disease.
The difference is dramatic. In one analysis of 1,000 trio cases, the diagnostic rate reached 47%, compared to 34% for patients tested alone. Even patients who had already undergone targeted gene panel testing still had a 30% chance of getting a diagnosis through trio exome sequencing. Trio analysis also speeds up interpretation, because comparing the patient’s variants against parental data immediately eliminates thousands of harmless inherited differences, narrowing the search considerably.
How It Compares to Other Genetic Tests
Exome sequencing sits between two other common approaches: targeted gene panels and whole genome sequencing.
- Targeted gene panels test anywhere from 2 to over 1,000 specific genes already linked to a known condition. They’re the cheapest option and provide the deepest reading of each gene, which makes them good at catching certain subtle mutations. The tradeoff is that they can only find problems in genes already on the list. If your condition is caused by a gene nobody has connected to disease yet, a panel will miss it.
- Whole exome sequencing covers all 20,000 protein-coding genes. It can identify new genetic causes of disease that haven’t been catalogued before, and it doesn’t need to be updated as new gene discoveries are made. It does miss mutations hidden in non-coding regions of DNA, and its reading depth is shallower than panels, so it’s less reliable at detecting mosaicism (where only some of your cells carry a mutation).
- Whole genome sequencing (WGS) reads nearly everything, including the non-coding 98.5%. It’s the best option for detecting large structural rearrangements and copy number changes in DNA. But it generates vastly more data, costs more, and has the highest chance of turning up variants whose significance is unclear.
What Results Look Like
Exome results typically fall into a few categories. The clearest outcome is a “positive” result: the lab identifies a variant in a gene that’s known to cause a specific condition, and it matches the patient’s symptoms. This can end a years-long diagnostic journey, open the door to targeted treatments, and help families understand recurrence risk for future children.
A “negative” result means no disease-causing variant was found. That doesn’t necessarily mean the cause isn’t genetic. It may sit in a non-coding region the exome doesn’t cover, or in a type of structural change that exome sequencing struggles to detect.
The trickiest outcome is a “variant of uncertain significance,” or VUS. This means the lab found a genetic change that looks suspicious but doesn’t have enough evidence yet to be classified as harmful or harmless. VUS results can sometimes be reclassified over time as more research accumulates, and labs may re-contact patients when this happens.
Secondary Findings
Because exome testing reads so many genes, it occasionally uncovers mutations unrelated to the reason the test was ordered. For example, a child tested for developmental delay might turn out to carry a mutation linked to a heart rhythm disorder or cancer predisposition.
The American College of Medical Genetics and Genomics maintains a curated list of 84 genes (as of the 2025 update) that labs are recommended to check and report on, regardless of the original reason for testing. These genes are associated with conditions where early detection can meaningfully change outcomes, such as hereditary breast cancer syndromes or familial high cholesterol. Patients can opt out of receiving these secondary findings before the test is run.
Cost and Turnaround Time
Clinical exome sequencing typically costs between $1,700 and $2,500 per sample, depending on whether the lab charges internal or external rates. Trio testing multiplies that cost by three, though some labs offer bundled pricing. Insurance coverage varies widely. Many insurers now cover exome testing for patients who meet specific criteria, usually after smaller gene panels have come back negative.
Standard turnaround times range from about 11 to 21 weeks, with 15 to 18 weeks being typical at major academic centers. Some labs have developed expedited protocols that deliver results in roughly 40 days. For critically ill newborns, rapid whole genome sequencing (a related but different test) can now return results in as little as 50 hours, though this isn’t standard for most exome orders.
Limitations Worth Knowing
Exome testing is powerful but not comprehensive. It misses mutations in the 98.5% of the genome that doesn’t code for proteins, including regulatory regions that control when and how much of a gene is turned on. It has limited ability to detect large deletions, duplications, or rearrangements of chromosomes. And its shallower reading depth means it may not catch mutations present in only a fraction of cells.
The interpretation of exome data also depends heavily on current medical knowledge. A variant might be classified as “uncertain” today and reclassified as disease-causing five years from now, as researchers study more patients. For this reason, some genetics teams recommend periodic reanalysis of exome data, especially if symptoms evolve or new genes are linked to a patient’s condition.