A negative exome or genome result does not automatically exclude a repeat-expansion disorder. The important question is whether the laboratory analysed the relevant repeat, using a method validated for its sequence and size range. This guide explains how to read that part of a report, why detecting an expansion is different from measuring it precisely, and when another test can add information.
What is a repeat expansion?
A short DNA sequence can occur several times in succession. At some locations, a disease-associated expansion changes gene activity or the properties of the gene product. The repeat’s location, sequence and architecture matter: a count from one gene cannot be interpreted using another gene’s thresholds. DMPK-related myotonic dystrophy type 1 involves CTG repeats, whereas FMR1-related disorders involve CGG repeats. Their clinical interpretation and additional testing requirements differ. ACMG DMPK testing standard; GeneReviews: FMR1 disorders.
Think of a repeated sequence as a passage containing the same short word over and over. A small fragment from the middle may identify the word without telling you how many times it appears in the complete passage. Unique sequences on either side help place the repeat in the genome.
Three claims that should not be confused
| Report statement | What it may establish | What to ask next |
|---|---|---|
| An expansion signal was detected | Evidence suggesting an expanded allele | Was this a screening call or a confirmed clinical result? |
| An estimated repeat size was reported | A size estimate within the method’s capabilities | What is the uncertainty, and can very large alleles be underestimated? |
| A disease-associated allele was characterised | Enough validated information for a locus-specific interpretation | Were other relevant features, such as methylation or interruptions, assessed? |
Short reads do not have to span the entire expansion to provide evidence for it. ExpansionHunter demonstrated how reads within repeats and at their boundaries can support expansion detection. That does not make an inferred size equivalent to directly reading one complete allele from end to end. Dolzhenko and colleagues, 2017.
What does the clinical validation evidence show?
A UK study published in 2022 compared genome-based calling with PCR results across 13 neurological repeat loci. Before additional visual curation, the analysis correctly classified 215 of 221 expanded alleles and 1,316 of 1,321 non-expanded alleles. These are encouraging results for the tested workflow and loci, not a guarantee that every expansion disorder is excluded by every genome test. Large expansions at some loci could be underestimated. Original diagnostic validation study.
For researchers, record the unit of analysis: those counts refer to alleles, not independent patients with a proven diagnosis. Also distinguish classification of an allele as expanded from agreement on its exact size. A single headline accuracy percentage can hide that difference.
Four practical report-reading examples
Fictional example: DMPK appears on a negative panel. A clinician sees DMPK in a gene list and assumes myotonic dystrophy type 1 has been excluded. The methods section, however, describes sequence-variant analysis without a validated CTG expansion assay. The useful next step is to ask the laboratory whether the repeat was actually tested. A gene name on a panel is not a promise to detect every variant type in that gene.
Fictional example: a large expansion has a precise-looking number. A genome analysis flags an expansion and estimates its size. Before using that number for counselling, the team checks the measurement range and confirmatory method. The number’s appearance in a spreadsheet does not remove analytical uncertainty.
Fictional example: an FMR1 result needs another layer. A family receives a CGG repeat result and asks whether repeat count explains the entire situation. The laboratory clarifies whether methylation, mosaicism or AGG interruptions are relevant to the particular clinical or reproductive question, and which were assessed. These features are not interchangeable, and not every person needs every assay. FMR1 testing and counselling framework.
Fictional example: one peak on a conventional PCR test. One observed DMPK allele could reflect two same-sized alleles; a large allele may also fail to amplify with a limited method. Interpretation depends on the complete validated workflow, including whether repeat-primed PCR or another method was used. The ACMG standard describes why methods and detection limits must be stated. 2024 ACMG technical standard.
Where does long-read sequencing fit?
Long reads can help resolve repeat length and sequence architecture when the data cover the relevant region adequately. Some workflows can also examine methylation. “Long-read” remains a description of a technology, however, rather than evidence that a particular laboratory has validated every repeat, tissue context and reporting category. Ask what the assay actually demonstrated for the suspected disorder. Read our long-read sequencing guide for practical study-design questions.
A checklist to take back to the laboratory
- Name the suspected disorder and exact repeat locus.
- Confirm that repeat-expansion analysis was included, rather than sequence analysis alone.
- Record the specimen, method and reportable size range.
- Distinguish a screening flag, estimated size and confirmed interpretation.
- Ask about interruptions, methylation or mosaicism only where relevant to that locus.
- Clarify what a negative result leaves unresolved and whether another assay is justified.
Use the DMPK guide and FMR1 guide for condition-specific interpretation. Our testing-limitations guide and research worksheet help keep the method, evidence and remaining question together.