Most cases of ALS are not directly inherited. Roughly 90 to 95% of people diagnosed with ALS have no family history of the disease. The remaining 5 to 10% have what’s called familial ALS, meaning at least one other family member has also been affected. But the line between inherited and non-inherited ALS is blurrier than those numbers suggest, because genetic mutations play a role in both forms.
Familial vs. Sporadic ALS
ALS is split into two broad categories. Familial ALS (fALS) refers to cases where the disease runs in the family. Sporadic ALS (sALS) means it appeared without any known family history. A large meta-analysis of 165 studies found that familial cases make up about 8% of all ALS diagnoses, though individual estimates range widely depending on how strictly “family history” is defined.
This distinction matters for understanding risk, but it isn’t as clean as it sounds. When researchers genetically test people classified as having sporadic ALS, about 11% turn out to carry a known disease-linked mutation. That means roughly 1 in 9 people told they have the “non-inherited” form actually carry a genetic variant that may have caused or contributed to their disease. In many of these cases, the mutation arose spontaneously (called a de novo mutation) or was passed down through family members who never developed symptoms.
The Major Genes Involved
More than 30 genes have been linked to ALS, but four account for the vast majority of known genetic cases. In familial ALS, these four genes together explain about 48% of cases:
- C9orf72: The most common ALS gene, responsible for about 22.5% of familial cases and 3 to 5% of sporadic cases. This mutation involves a short DNA sequence that repeats far more times than it should. It’s also strongly linked to frontotemporal dementia, and some carriers develop both conditions.
- SOD1: The first ALS gene ever discovered, accounting for about 19% of familial cases. SOD1 mutations cause the body to produce a malfunctioning version of a protein that normally protects cells from damage. This gene is notable because it’s the target of one of the first gene-specific ALS treatments.
- TARDBP: Found in roughly 3.3% of familial cases. Mutations in this gene cause a protein called TDP-43 to misfold and clump inside motor neurons.
- FUS: Also about 3% of familial cases. Like TARDBP, FUS mutations cause a protein to accumulate abnormally in nerve cells.
In the remaining familial cases where these four genes aren’t involved, rarer mutations in genes like OPTN, NEK1, TBK1, and others may be responsible. For about one-third of familial ALS patients, no genetic cause has been identified yet.
How ALS Is Passed Down
When ALS is inherited, the pattern depends on which gene is involved. The most common pattern is autosomal dominant, which means inheriting just one copy of the mutated gene (from either parent) is enough to cause the disease. C9orf72, TARDBP, and most SOD1 mutations follow this pattern. If a parent carries one of these mutations, each child has a 50% chance of inheriting it.
Some rarer forms follow an autosomal recessive pattern, meaning a person needs to inherit the mutation from both parents to be affected. This is more common in juvenile ALS, which begins before age 25. The genes ALS2 (alsin), SPG11, and SIGMAR1 all follow recessive inheritance and tend to cause disease in childhood or adolescence. At least one form, linked to the UBQLN2 gene, follows X-linked inheritance, meaning it’s carried on the X chromosome and affects males and females differently.
SOD1 and FUS are unusual in that they can follow either dominant or recessive patterns depending on the specific mutation involved. This is one reason why genetic counseling, not just a simple test result, is important for understanding family risk.
Carrying a Mutation Doesn’t Guarantee Disease
One of the most important concepts for families is penetrance: the likelihood that someone who carries a mutation will actually develop symptoms. Not every carrier gets sick, and timing varies enormously.
The C9orf72 expansion offers the clearest data. Among carriers, the median age when symptoms first appear is 58. Penetrance rises steadily with age and reaches nearly 100% by the early 80s. That means some carriers live into their 60s or 70s without symptoms, while others develop disease in their 40s. When researchers use more conservative statistical models, lifetime penetrance drops to around 91%, suggesting that a small fraction of carriers may never develop the disease at all.
Penetrance data for other genes is less complete, partly because individual mutations are rarer and harder to study in large numbers. But the general principle holds: carrying an ALS-linked mutation raises risk significantly without making disease inevitable in every case.
Sporadic ALS and Hidden Genetics
Even when there’s no family history, genetics often plays a role. A large study of patients diagnosed with sporadic ALS found that about 5.2% carried the C9orf72 expansion, and another 5.6% carried harmful mutations in other known ALS genes. Beyond those clearly damaging variants, an additional 17% carried genetic susceptibility factors that may increase risk without directly causing disease on their own.
Still, in about 72% of sporadic cases, no genetic explanation was found. This doesn’t necessarily mean genes aren’t involved. It may mean the relevant genes haven’t been discovered yet, or that the disease results from a combination of multiple small genetic effects interacting with environmental factors. ALS is increasingly understood as a disease with a genetic spectrum rather than a simple inherited-or-not divide.
Genetic Testing and Who Should Consider It
Current clinical guidelines recommend that all people diagnosed with ALS be offered genetic counseling and testing for at least the four major genes: C9orf72, SOD1, FUS, and TARDBP. This applies regardless of family history, because mutations show up in both familial and sporadic cases, and identifying a specific mutation can affect treatment options.
For people who don’t have ALS but are concerned about family risk, testing is generally recommended when a parent or sibling has a confirmed ALS-related mutation, or when one or more first-degree relatives have been diagnosed with ALS even without a known genetic cause. People of reproductive age in these situations may also pursue preconception genetic testing to understand the risk of passing a mutation to their children.
Deciding whether to get tested when you’re healthy but at risk is deeply personal. A positive result can mean decades of uncertainty, since penetrance isn’t 100% and no proven prevention exists. Genetic counselors can help weigh the psychological impact against the practical benefits, which include eligibility for clinical trials and the ability to plan ahead.
What Genetic Type Means for Prognosis
People often wonder whether inherited ALS progresses differently than the sporadic form. Among patients with SOD1 mutations, studies show no significant difference in disease progression or survival between those with a family history and those without. Mean survival was roughly 7 to 10 years for both groups. This suggests that the biological effects of a given mutation matter more than whether it was inherited or arose spontaneously.
That said, different genes and even different mutations within the same gene can produce very different disease courses. Some SOD1 mutations are associated with slow progression over a decade or more, while others lead to rapid decline. C9orf72 carriers tend to develop symptoms in their late 50s and frequently have cognitive or behavioral changes alongside motor symptoms. Juvenile forms linked to recessive genes like ALS2 often progress more slowly than typical adult-onset ALS but begin much earlier in life.
Knowing the specific genetic variant, when one exists, gives families and clinicians a clearer picture of what to expect and which therapies might be most relevant.