A frameshift mutation is a change in DNA where one or more nucleotides are inserted into or deleted from a protein-coding sequence in a number that is not a multiple of three. Because cells read DNA in three-letter groups to build proteins, adding or removing even a single nucleotide throws off the entire downstream reading, much like removing one letter from a long sentence would scramble every word that follows. The result is usually a completely different, nonfunctional protein, or no protein at all. Frameshifts sit behind some of the most severe inherited diseases in medicine, but they also play surprising roles in viral replication, cancer immunotherapy, and even the evolution of new genes.
Why Three Is the Magic Number
The discovery that the genetic code is read in triplets came directly from experiments with frameshift mutations. In 1961, Francis Crick and Sydney Brenner used a chemical called proflavin to insert or delete individual bases in a bacteriophage gene. When they added or removed one or two bases, the gene stopped working. But when they added or removed exactly three bases, the gene often still produced a functional protein.1Natural Sciences. A breakthrough from 60 years ago: “General nature of the genetic code for proteins” (1961) That elegant result proved the code was triplet-based: knock it off by one or two, and every downstream codon misreads; knock it off by three, and the reading frame recovers with only a small stretch of altered amino acids. Frameshifts, in other words, were the tool that cracked the code itself.
How Frameshifts Happen
Most frameshift mutations arise when the machinery that copies DNA slips during replication. Stretches of repetitive sequence are especially prone to this: when the same short motif repeats several times in a row, the copying enzyme can lose its place, skip ahead, or loop back, adding or dropping a base or two in the process. In bacteria, these misalignment errors at repetitive DNA sequences are one of the most common sources of genetic variation.2PubMed. Encoded errors: mutations and rearrangements mediated by misalignment at repetitive DNA sequences The same principle applies in human cells. Certain chemical mutagens and radiation can also cause insertions or deletions, but replication slippage at repeat tracts is the workhorse mechanism.
This matters clinically because some regions of the genome are natural hotspots. Short tandem repeats inside protein-coding genes are especially vulnerable when the cell’s mismatch repair system is faulty, a situation that becomes central in certain cancers.
What the Cell Does With a Broken Message
A frameshift almost always creates a premature stop signal somewhere in the mangled downstream sequence. The cell has a quality-control system designed to catch exactly this problem. When the protein-building machinery encounters a stop signal that appears too early, the messenger RNA carrying those instructions is flagged and destroyed before much protein is made. Researchers studying a frameshift mutation in the heart-rhythm gene hERG confirmed that the mutant messenger RNA was actively degraded through this quality-control pathway; blocking the pathway caused the mutant message to accumulate and even produce some of the altered protein.3PubMed Central. Nonsense-Mediated mRNA Decay Caused by a Frameshift Mutation in a Large Kindred of Type 2 Long QT Syndrome The same cleanup process has been documented for a frameshift in the alpha-globin gene that causes a form of thalassemia: the premature stop codon triggered rapid destruction of the message.4PubMed. Human alpha2-globin nonsense-mediated mRNA decay induced by a novel alpha-thalassaemia frameshift mutation at codon 22
A particularly striking example comes from a movement disorder called paroxysmal kinesigenic dyskinesia. A frameshift mutation in the PRRT2 gene produced a mutant message with roughly a tenth of the normal stability: the messenger RNA half-life was dramatically shortened, and the resulting protein was undetectable in mouse brain tissue.5Biochem Biophys Res Commun. PRRT2 frameshift mutation reduces its mRNA stability resulting loss of function in Paroxysmal Kinesigenic Dyskinesia In all these cases, the cell’s surveillance system is doing its job: catching the corrupted message early and preventing a garbled protein from causing further damage.
When the Cleanup Fails
The quality-control system has a blind spot. It generally does not catch premature stop codons that fall in the last exon or near the very end of a gene, because there is no downstream signal to flag the message as abnormal. When a frameshift mutation lands in this zone, the garbled protein gets made and can accumulate inside cells. Researchers studying the myelin protein zero gene (MPZ), which is linked to inherited peripheral neuropathies, found that frameshift mutations escaping the surveillance pathway caused mutant protein to pile up in the endoplasmic reticulum, triggering cell death and severe disease.6American Journal of Human Genetics. Functional Consequences of MPZ-Truncating Mutations that Escape Nonsense-Mediated Decay Some other truncating mutations in the same gene did get caught by the surveillance system and were associated with a much milder form of neuropathy. The location of the frameshift within the gene, then, can be the difference between a mild condition and a devastating one.
There are also contexts where other mechanisms allow a message to dodge the surveillance pathway. Ribosomes can sometimes read through a premature stop codon, slip back into the original reading frame, or reinitiate translation downstream, all of which can let an otherwise doomed transcript survive.7Journal of Biological Chemistry. Features and factors that dictate if terminating ribosomes cause or counteract nonsense-mediated mRNA decay These escape routes are rare, but they explain some of the puzzling variation in how different people respond to what appear to be similar mutations.
Duchenne Versus Becker Muscular Dystrophy
The clearest clinical illustration of what frameshifts do comes from the dystrophin gene, one of the largest genes in the human genome. When a deletion in this gene shifts the reading frame, the protein is cut short and nonfunctional, and the result is Duchenne muscular dystrophy, a severe condition that typically confines boys to wheelchairs by their early teens. When a deletion removes one or more exons but keeps the reading frame intact, a shorter but partially functional dystrophin protein is made, and the result is the milder Becker muscular dystrophy. This “reading-frame rule” was proposed in the late 1980s, and it holds true for about 91% of the more than 4,700 mutations catalogued in the Leiden DMD mutation database.8PubMed. Entries in the Leiden Duchenne muscular dystrophy mutation database: an overview of mutation types and paradoxical cases that confirm the reading-frame rule An independent analysis of 258 deletions found a 92% agreement rate.9PubMed Central. The molecular basis for Duchenne versus Becker muscular dystrophy: correlation of severity with type of deletion
The remaining 8 to 9% of cases that break the rule are instructive. Some patients with frame-disrupting deletions have milder Becker phenotypes because their cells find workarounds, such as alternative splicing that skips the troublesome region and restores the frame downstream. Others with in-frame deletions still develop severe Duchenne because the deleted region happened to encode a part of the protein that is structurally indispensable. The reading-frame rule is genuinely useful for early prognosis, but it is not destiny.10PubMed Central. Walking alone milestone combined reading-frame rule improves early prediction of Duchenne muscular dystrophy
Frameshifts in Cancer
In hereditary breast and ovarian cancer, some of the most studied mutations in the BRCA1 and BRCA2 genes are frameshifts. A meta-analysis across 29 studies identified 20 common founder mutations in these genes; several of the most frequently reported were small insertions or deletions that shift the reading frame, including the well-known BRCA1 5382insC and 185delAG mutations.11PubMed. Common BRCA1 and BRCA2 mutations in breast cancer families: a meta-analysis from systematic review In a Bulgarian cohort of high-risk breast cancer patients, frameshifts accounted for a substantial share of the pathogenic mutations found across both genes.12PubMed Central. Spectrum and frequencies of BRCA1/2 mutations in Bulgarian high risk breast cancer patients A population-based study in Iceland found that female carriers of the BRCA2 999del5 frameshift mutation had an estimated breast cancer risk of about 17% by age 50 and roughly 37% by age 70.13The Lancet. Population-based study of breast cancer risk associated with BRCA2 mutation
Frameshifts also play a different and fascinating role in cancers with defective mismatch repair. When a tumor loses the ability to fix replication errors, frameshift mutations pile up at coding microsatellites throughout the genome.14PubMed. The Immune Biology of Microsatellite-Unstable Cancer In microsatellite-unstable colorectal cancers, for example, about 39% of primary tumors carried a frameshift in the TCF-4 gene, compared to fewer than 2% of microsatellite-stable tumors.15Cancer Research. Frequent Frameshift Mutations of the TCF-4 Gene in Colorectal Cancers with Microsatellite Instability These tumors carry a heavy load of frameshifts across many genes simultaneously, and many of the same frameshifts recur across different patients.16PubMed Central. The shared frameshift mutation landscape of microsatellite-unstable cancers suggests immunoediting during tumor evolution
Frameshifts and Immunotherapy
Here is where frameshifts get interesting from a treatment standpoint. A frameshift produces a stretch of amino acids that the body has never seen before, because the entire downstream sequence is scrambled. That means the resulting protein fragments look foreign to the immune system, making them strong potential targets for immune attack. In non-small-cell lung cancer, patients whose tumors carried frameshift insertions or deletions responded to immune checkpoint inhibitors at roughly double the rate of patients whose tumors lacked them: overall response rates of about 26% versus 12%.17PubMed. Clinical and Immunological Implications of Frameshift Mutations in Lung Cancer
Researchers have taken this a step further by identifying shared frameshift-derived peptides across microsatellite-unstable cancers and testing whether they actually trigger immune responses. Selected frameshift peptides turned out to be highly immunogenic, generating strong immune cell responses in both healthy donors and patients with microsatellite-unstable tumors.18Cell. Frameshift Mutations in Microsatellite-Unstable Cancers Give Rise to Shared Neoantigens and Effective T Cell Responses Because many of these frameshifts recur across patients, they could form the basis for off-the-shelf cancer vaccines that would not need to be customized for each individual tumor. The evidence is still early, but the logic is compelling: frameshifts are so disruptive to the protein sequence that the immune system has an easy time recognizing them as abnormal.
Programmed Frameshifting in Viruses and Human Cells
Not all frameshifts are accidents. Many viruses deliberately use frameshifting as a gene-regulation strategy. By embedding special signals in their RNA, viruses cause the ribosome to slip backward or forward by one nucleotide at a precise location, producing a different protein than the one the original reading frame would encode. This lets viruses make two different proteins from a single stretch of genetic material while controlling the ratio between them.19PubMed Central. Regulators of Viral Frameshifting: More Than RNA Influences Translation Events HIV and SARS-CoV-2 both depend on this trick for their replication.20Nature Communications. High-throughput interrogation of programmed ribosomal frameshifting in human cells
Structural studies of SARS-CoV-2 have shown exactly how it works: the viral RNA folds into a knot-like structure that jams at the entrance to the ribosome’s channel, creating tension that forces the ribosome to slip one position backward. This slip allows the virus to produce its RNA-copying enzyme and all the downstream proteins it needs to replicate.21PubMed Central. Structural basis of ribosomal frameshifting during translation of the SARS-CoV-2 RNA genome Because this frameshifting event is essential for the virus, it has become a drug target: if you could jam or disrupt the RNA structure that forces the slip, you would block viral replication without touching human cells.
Human cells use programmed frameshifting too, though far less commonly. The best-studied example involves antizyme, a protein that helps regulate the levels of polyamines, small molecules involved in cell growth. Antizyme is encoded across two overlapping reading frames, and producing the full protein requires the ribosome to slip forward by one nucleotide at a specific site. The clever part: polyamines themselves stimulate this slip, so when polyamine levels rise, more antizyme is made, which in turn suppresses polyamine production. It is a built-in feedback loop controlled by frameshifting.22Cell. Ornithine Decarboxylase Antizyme Is Expressed by Functional Ribosomal Frameshifting 23PubMed Central. Translational recoding as a feedback controller: systems approaches reveal polyamine-specific effects on the antizyme ribosomal frameshift A second mammalian antizyme uses the same polyamine-stimulated frameshifting mechanism, suggesting the strategy has been conserved over a long evolutionary span.24PubMed. A second mammalian antizyme: conservation of programmed ribosomal frameshifting
Frameshifts as Engines of Evolution
Frameshifts are usually discussed as disasters, and in most individual cases they are. But over evolutionary timescales, they have occasionally served as raw material for creating entirely new proteins. A survey of vertebrate gene families found that when a second copy of a gene exists to keep producing the original protein, a frameshift in one copy can persist for millions of years and eventually give rise to a protein with a genuinely new function.25PubMed. Functional divergence of proteins through frameshift mutations A separate computational study identified hundreds of apparent frameshift events in human and mouse genes that generated new protein-coding segments, with very little overlap between the two species, suggesting these events accumulate rapidly on an evolutionary timescale.26PubMed. Frequent appearance of novel protein-coding sequences by frameshift translation
On the other end of the spectrum, frameshifts are the primary mechanism by which functional genes decay into pseudogenes in the tuberculosis-causing bacterium Mycobacterium tuberculosis and its relatives. Frameshifts outpaced all other types of gene-inactivating mutations in driving pseudogene formation across the entire complex of related mycobacterial species.27PubMed Central. The rate and role of pseudogenes of the Mycobacterium tuberculosis complex In bacteria that live inside a host and face less diverse environmental pressure, losing certain genes through frameshifts may actually be neutral or even beneficial, streamlining the genome over time.
Therapeutic Strategies That Fix the Frame
The reading-frame rule behind Duchenne muscular dystrophy has inspired one of the most creative gene-therapy strategies in clinical medicine: exon skipping. The idea is simple in concept. If a deletion has thrown the reading frame off, you use a short synthetic molecule called an antisense oligonucleotide to make the cell’s splicing machinery skip over a neighboring exon, removing it from the final message. If you pick the right exon to skip, the downstream reading frame is restored, and the cell produces a shorter but partially functional dystrophin protein, converting what would be severe Duchenne into something closer to milder Becker.28PubMed Central. Antisense-mediated exon skipping: a versatile tool with therapeutic and research applications 29PubMed Central. Antisense Oligonucleotide-Mediated Exon-skipping Therapies: Precision Medicine Spreading from Duchenne Muscular Dystrophy Several exon-skipping drugs have now been approved for specific subsets of Duchenne patients, each targeting a different exon.
The same approach is being explored for cystic fibrosis. Researchers designed antisense oligonucleotides to skip exon 23 of the CFTR gene, which contains the W1282X mutation, the fifth most common cystic fibrosis mutation. In airway cells from a patient carrying that mutation, exon skipping rescued both CFTR protein expression and chloride channel function.30PubMed Central. Open reading frame correction using splice-switching antisense oligonucleotides for the treatment of cystic fibrosis The strategy is not yet in clinical use for cystic fibrosis, but it illustrates how broadly the principle can be applied whenever a truncating mutation lies within a skippable exon.
Gene editing offers a more permanent alternative. CRISPR-based tools can introduce precise insertions to correct a frameshift at the DNA level. One challenge has been controlling exactly what kind of edit the cell makes after CRISPR cuts the DNA. Recent work has shown that co-delivering a modified version of T4 DNA polymerase alongside Cas9 can dramatically bias the repair outcome toward one-base-pair insertions, achieving up to about 72% single-nucleotide insertions at certain genomic sites while keeping undesired edits below 5%.31PubMed Central. T4 DNA polymerase prevents deleterious on-target DNA damage and enhances precise CRISPR editing A precise one-base insertion at the right location could, in theory, restore the reading frame of a gene that had been disrupted by a one-base deletion.
How Suppressor tRNAs Override a Frameshift
Bacteria have yet another way to deal with frameshifts: suppressor transfer RNAs. These are mutant forms of the adapter molecules that bring amino acids to the ribosome. A normal tRNA reads a three-letter codon, but a suppressor tRNA has a modified structure that causes the ribosome to read four letters instead of three, effectively absorbing the extra nucleotide caused by an insertion and putting the reading frame back on track. Structural studies have shown that these suppressor tRNAs distort the shape of the ribosome’s small subunit, loosening its grip on the message and allowing the reading frame to shift.32PubMed Central. Mechanism of tRNA-mediated +1 ribosomal frameshifting A different suppressor tRNA achieves the same result through a slightly different structural mechanism, tilting within the ribosome in a way that disrupts normal frame maintenance.33PubMed Central. Structural insights into translational recoding by frameshift suppressor tRNASufJ
These suppressor tRNAs are mostly laboratory tools and natural curiosities in bacteria, but they have informed the broader understanding of how ribosomes maintain the reading frame in all organisms. The ribosome is not passively sliding along the message; it is actively gripping the RNA and holding the frame in place. Anything that disrupts that grip, whether a suppressor tRNA, a viral pseudoknot, or a polyamine-responsive signal, can cause the frame to shift. Understanding those mechanisms has opened the door to engineering frameshifts on purpose, both for research and, potentially, for therapy.