Exome vs. Genome: What’s the Difference Between Them?

Your genome is all of your DNA. Your exome is the roughly 1 to 1.5% of it that holds the instructions for making proteins, which is where most known disease-causing changes sit. An exome test reads only that part, while a genome test reads nearly everything. The genome test can catch some changes the exome test misses, but for many families the chance of an answer is similar, and the exome test costs less. The bigger practical differences show up in what each test can miss and in what happens when the first result comes back empty.

What an Exome Test Reads and What It Skips

The exome works out to roughly 30 million DNA letters, out of about 3 billion in a full genome.¹² On a test order you may see exome sequencing written as ES or WES, and genome sequencing as GS or WGS. Both tests read the same DNA, taken from the same blood or saliva sample.

That other 98 to 99% is not filler. Much of it contains switches that turn genes on and off, and some parts help cut and paste a gene’s message correctly. A change there can cause disease even when the protein instructions look normal. Scientists understand far less about this region, so a finding in it is harder to interpret.

For an exome test, the lab uses molecular “bait” to fish out the protein-coding pieces before reading them, which cuts cost.³ A genome test skips that selection and reads DNA from across the whole sample. The fishing step does not grab every piece equally well, while a genome test reads DNA more evenly.⁴

The machines produce millions of short fragments of sequence, and software lines them up against a reference human genome to see where yours differs. Everyone differs from the reference at millions of spots, so the real work is deciding which few differences could explain the symptoms. Even in a genome test, stretches rich in the letters G and C, or with many repeats, are harder to read.⁵

Each position is read many times over so random reading errors can be told apart from true differences. Where a region is read too few times, the lab cannot be confident about it, and that spot may be reported as not adequately covered. It’s worth asking the lab whether the genes most relevant to your condition were read well, since a gap in exactly those genes matters more than gaps elsewhere.

Which Test Is More Likely to Find a Diagnosis

In a trial of over 1,000 people with suspected rare genetic diseases, each test found an answer for about 34% of patients.⁶ Pooled studies of children with rare diseases found genome tests solved about 31% of cases versus 23% for exome tests.⁷ That gap was not certain enough to rule out chance, so it is best read as a possible edge for the genome test rather than a proven one.⁸ When genome testing was the first test, it tended to find answers more often across different kinds of conditions.⁹

“Finding an answer” means a DNA change was found that explains the symptoms. Most people tested, about two in three, still get no clear answer. A negative result does not prove the condition is not genetic. Some results come back labeled “uncertain,” meaning the meaning of the change is not yet known. A figure like 34% is an average across many different patients, so it cannot predict what will happen for one particular child or adult.

For you, the odds improve with certain features. People with symptoms in several body systems, especially starting in infancy or childhood, are more often tested and more often solved than people with a single mild symptom. A family history of the same condition can point the lab toward particular genes. Conditions driven mostly by many small genetic and environmental factors, rather than one gene, are less likely to be explained by either test. The clinical details your doctor sends with the sample guide the search, so a detailed symptom list can change whether a cause is spotted.

For children whose exome test came back empty, a genome test found an answer in about 24%.¹⁰ But simply re-examining the original exome data found answers in about 14%, a statistically similar result.¹¹ A negative exome does not automatically mean a genome test is the next step.

What an Exome Test Can Miss

DNA changes outside the coding parts can still disrupt how a gene works.¹² Some cases of beta-thalassemia, a blood disorder, come from changes deep inside a gene’s non-coding stretches.¹³ Some familial high cholesterol comes from faulty gene switches, and some inherited eye diseases also arise this way.¹⁴ In one clinic’s solved genome cases, 43% hinged on large rearrangements or non-coding changes.¹⁵

A person can have an exome result that looks normal because the problem sits just outside the part that was read. That’s why a negative exome test does not rule out a mutation in a gene the doctor already suspects.

A large rearrangement is like a paragraph removed, copied, flipped, or pasted in the wrong place, rather than a single misspelled letter. Deleted or duplicated chunks can be spotted from exome data in many cases.¹⁶ They leave a clue, because the region is read fewer or more times than expected. In a large children’s study, exome-based calling caught about 87 to 95% of confirmed deleted or duplicated chunks.¹⁷

Flipped or moved pieces of DNA are harder, and even genome tests miss many mid-sized changes that newer long-read tests catch.¹⁸¹⁹ They leave no count clue, so they usually have to be found by reading across the break points, and the exome’s gaps between targeted regions make that difficult. Even with a genome test, detection depends on the lab’s software and method, so ask whether rearrangement analysis is included in the report.

Disorders caused by a stretch of DNA repeating too many times, such as Fragile X, can be screened for in both exome and genome data.²⁰²¹ Screening is a specialized analysis, though, and a lab may not run it unless asked. If a specific repeat condition is suspected, a dedicated single-purpose test may be the more direct route, and the doctor can say whether it makes sense.

Mitochondria, the cell’s power units, carry a small amount of their own DNA, separate from the main set in the nucleus. This DNA is passed down through the mother, so the pattern in a family can differ from other inherited conditions. Standard exome kits do not target it,²² while a genome test captures it as part of reading everything. If a mitochondrial disease is suspected, ask whether the lab uses a kit that covers mitochondrial DNA.

At least 32 medicines depend on gene variants that standard exome tests cannot read reliably.²³ Don’t assume an exome test checked how you process common drugs. Missing drug-response results can look like “normal” when they were never read, and a dedicated drug-response gene panel is the more reliable choice for this.

Given all this, a doctor might prefer a genome test when an earlier exome test was negative and symptoms strongly suggest a genetic cause. The same goes for a condition known to come from non-coding or repeat changes, or when a mitochondrial disease is suspected and the lab’s exome kit skips mitochondrial DNA.

Cost and What You Get for It

Published prices per test ranged from about $555 to $5,169 for exome tests.²⁴ Genome tests ranged from about $1,906 to $24,810.²⁵ Many price estimates did not say what was included, such as interpretation or counseling.²⁶ A low quote may cover only the sequencing, while a higher one bundles in analysis, a written report, and counseling. Prices also differ by lab and by country, so the range tells you more about variation than about what you will pay. Part of the gap is data: a genome file takes about 120 to 150 gigabytes, versus about 6 to 8 for an exome.²⁷²⁸

Labs often test both parents alongside a child to see which changes are new in the child. This “trio” approach makes results easier to interpret. It can raise the price but improve the chance of an answer.

Your out-of-pocket cost depends on insurance, which may require prior approval and may cover one test but not the other. Ask the ordering office for a written cost estimate, and whether the lab offers financial assistance if coverage is denied. Also ask whether the price includes interpretation, counseling, and parent samples, and whether future reanalysis is included or billed separately. Genome prices have been falling, so current quotes may beat older figures. A cheaper test that leaves the question unanswered can end up costing more if a second test follows, so weigh the price against what each test can reach.

What Happens if the Test Finds Nothing

A negative result is not necessarily final. A second look at existing exome or genome data finds new diagnoses in about 3 to 15% of cases.²⁹ It works because new disease genes are discovered and the meaning of known changes is updated over time. The DNA itself has not changed, only the knowledge used to read it, which is why no new sample is usually needed. One lab’s proactive rechecks turned about 5% of unsolved exome cases into diagnoses.³⁰ Rechecking genome data about 2 years later solved about 11% of previously unsolved cases.³¹ Experts suggest a fresh look every 1 to 2 years, or sooner if symptoms change.³² Ask your care team how and when your data will be rechecked.

To prompt a second look, contact the ordering doctor or genetic counselor rather than the lab directly. They can send the request along with updated clinical details. New symptoms, or a new diagnosis in the family, are worth reporting because they can point the lab toward different genes. Keep a copy of the original report and the lab’s name so any later team can find the data, and ask whether the lab contacts you automatically if a result is reclassified.

A second look is generally cheaper and faster than a new test, so it is often a sensible first move when the original data are good. A genome test can follow a negative exome, but a second look at the existing data may come first. A new test becomes more worthwhile if the first one could not read the kind of change suspected. Long-read genome tests read longer DNA stretches and catch hidden rearrangements, though they are less widely available and may need a referral to a specialized center. In the meantime, symptoms can be treated even without a known cause.

Findings You Didn’t Ask For

Both tests can turn up unrequested findings (your report may call them secondary findings): changes unrelated to why you were tested. Labs may report changes in a minimum list of genes tied to serious but preventable or treatable conditions.³³ Knowing about these risks lets doctors start screening or treatment early.³⁴ The results can also lead to more tests and worry for relatives who may carry the same change.³⁵

The listed genes are linked to conditions such as inherited heart rhythm problems or hereditary cancer risk, where early action can change outcomes. A finding means a risk, not a diagnosis, and many people with such a change never develop the condition. For a child, results can reveal that a parent carries the change, which is how a finding sometimes reaches the whole family. The listed genes are protein-coding, so both tests cover them, and what you receive depends on lab policy and your consent choices.

People often expect to be told important results even when the consent form says they won’t be.³⁶ Read the form for what will and won’t be returned, and ask before signing.

Deciding whether to receive unrequested findings comes down to a few questions. Would you want to know about a risk that is not causing symptoms now but could be managed? How might relatives feel, since the information may matter to them even though they did not choose the test? Opting out typically does not change the main answer about the original condition. A genetic counselor can go through these questions before you sign, which is easier than reversing a decision afterward.