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Every cell in the body carries the same complete set of genetic instructions, written in a long molecule called DNA. Only a small part of this instruction set, called the exome, is actually translated into the proteins that build and run the body; this coding portion makes up roughly 180,000 short stretches of DNA, called exons, spread across an estimated 20,000 genes, and altogether accounts for only about 1 to 2 percent of the complete genetic code. Whole exome sequencing is a laboratory test that reads this entire coding portion at once, rather than reading one gene, or even a handful of genes, at a time. It matters that the test focuses here because, even though the exome is a small fraction of the total genetic code, it is estimated to contain the great majority of genetic changes currently known to cause disease.
A doctor recommends whole exome sequencing when a person’s symptoms, or a pattern running through a family, point toward an underlying genetic cause that has not been identified, and when the number of genes that could plausibly be responsible is too large to test one at a time in any practical way. This comes up most often for children with unexplained developmental delay, intellectual disability, or several birth differences affecting more than one part of the body, where waiting to test genes individually, based on an educated guess, could take years and still not find an answer. Rather than guessing, whole exome sequencing casts a wide net across essentially all genes in one test, then narrows down to the change, or changes, that best explain what is actually being seen in that person.
It is worth understanding from the outset that whole exome sequencing is a diagnostic tool, not a guarantee of an answer. A clear genetic explanation is found in a meaningful proportion of cases, discussed further below, but not every family who undergoes this testing receives a definitive result on the first attempt. A geneticist or genetic counselor is generally involved both before testing, to set realistic expectations, and after, to interpret what the result actually means for the person and their family.
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