Whole Exome Sequencing (WES)
What It Is, How It Works, When to Order It

Whole exome sequencing (WES) is a genetic test that sequences all of the protein-coding regions of the human genome, collectively the exome. About 60 million base pairs spread across ~20,000 genes. That is roughly 1 to 2% of the total genome, but contains an estimated 85% of known disease-causing genetic variants.

Why the exome, not the whole genome?

The human genome contains ~3.2 billion base pairs, but most do not directly encode proteins. The protein-coding portions, the exons, are the segments cells actively read to produce the proteins that carry out virtually every biological function. Most variants causing Mendelian disease, disrupting enzyme function, altering structural proteins, or affecting signaling do so by changing exon sequence.

By focusing sequencing on the exome rather than the entire genome, WES captures the regions most likely to harbor disease-causing variants at a cost and data volume significantly lower than WGS. A standard clinical WES run produces ~10–20 GB of data per sample, roughly one-tenth the WGS data volume, while identifying the vast majority of clinically actionable variants.

What is an exon?

Exons are not contiguous. Each gene is made of multiple exons separated by non-coding introns. When a gene is expressed, the cell transcribes the entire gene (exons and introns together) and then splices out the introns to produce a mature mRNA consisting only of exons. That mRNA is translated into a protein. A variant in an exon that changes an amino acid, introduces a premature stop codon, or disrupts a splice site at the exon-intron boundary can alter or abolish protein function. Those are the variants WES is designed to detect.

WES follows the same general NGS workflow as other sequencing assays, with one critical addition: the exome capture step. Understanding the full pipeline helps clinicians interpret results and helps labs make informed decisions about test design.

Understanding both sides is essential for appropriate test ordering and accurate result interpretation. Being explicit about WES limitations protects patients from false reassurance and guides appropriate follow-up.

WES is not appropriate for every genetic testing scenario. It is most powerful when the clinical question is broad, the differential diagnosis spans many genes, and prior targeted testing has been unrevealing or is impractical.

One of the most powerful applications of WES is trio sequencing: sequencing the affected patient (proband) together with both biological parents simultaneously.

Trio WES dramatically improves diagnostic yield by enabling de novo variant detection. De novo variants are present in the child but absent from both parents, meaning they arose as new mutations, not inherited ones. De novo variants are a major cause of severe early-onset genetic conditions including autism spectrum disorder, severe intellectual disability, and many epileptic encephalopathies.

Without parental sequencing, a de novo variant looks like any other rare heterozygous variant. It must be evaluated on functional evidence alone. With parental sequencing, a de novo variant in a gene strongly linked to the child's phenotype carries substantially more evidence for pathogenicity, often meeting ACMG criteria for Pathogenic or Likely Pathogenic outright.

Trio WES also enables

• Compound heterozygosity confirmation: two variants in the same gene, one on each chromosome, that together cause autosomal recessive disease. Parental data confirms each variant is inherited from a different parent.
• X-linked inheritance confirmation: distinguishing maternal carrier inheritance vs de novo arising in an affected male, which changes recurrence risk counseling.
• Reduced VUS rate: parental segregation data helps reclassify variants of uncertain significance.

Studies consistently show trio WES achieves 10 to 15 percentage points higher diagnostic yield than proband-only WES for pediatric rare disease, a clinically significant difference that justifies the additional sequencing cost in most cases.

WES diagnostic yield varies by clinical indication, patient selection, and whether trio or proband-only sequencing is performed. The most reliable figures come from large clinical cohort studies.

Individual yield varies based on how well the phenotype is characterized, the specificity of the indication, and the lab's analytical approach. Highly specific phenotypes with strong genotype-phenotype correlations yield more diagnoses than broad presentations. A negative WES does not rule out a genetic cause: it rules out the causes detectable by WES. About 10 to 15% of WES-negative patients eventually receive a diagnosis through WGS, RNA sequencing, or re-analysis of existing WES data as new gene-disease relationships are established.

Clinical WES pricing in the United States varies by laboratory and testing configuration. These figures reflect typical clinical laboratory pricing inclusive of variant interpretation. Research-grade WES from commercial providers costs considerably less but excludes clinical reporting and CLIA-certified interpretation.

Insurance coverage for clinical WES has expanded substantially. Medicare covers WES for specific indications, primarily pediatric patients with suspected genetic conditions; coverage is governed by Local Coverage Determinations that vary by MAC region. Most commercial payers cover WES for children with unexplained developmental delay, intellectual disability, epilepsy, or multiple congenital anomalies when ordered by an appropriate specialist and prior testing has been non-diagnostic. Medical necessity documentation is typically required.