ALS is genetic in some cases, but most people who develop it have no family history of the disease. Roughly 5 to 10 percent of ALS cases are classified as “familial,” meaning the person inherited a gene mutation from a parent. The remaining 90 to 95 percent are considered “sporadic,” appearing without any obvious genetic link. But that clean division is misleading, because genetics plays a broader role than those numbers suggest.
Familial vs. Sporadic ALS
The traditional estimate is that about 10 percent of ALS cases run in families. A long-running study of ALS incidence in Ireland found a mean familial rate of 11.1 percent, consistent with figures seen across populations of European ancestry. But the same research revealed that the true rate is likely at least 20 percent, and possibly as high as 30 percent when researchers look at extended family patterns, including relatives with related conditions like frontotemporal dementia.
The gap between the traditional 10 percent figure and the higher estimates comes down to how “familial” is defined. Many families don’t realize ALS has appeared before, especially if a relative died decades ago without a clear diagnosis or if the family is small enough that the mutation simply never showed up in another member.
Genetics in “Sporadic” Cases
Even among people diagnosed with sporadic ALS, genetics is often involved. A large study published in Brain found that about 10.85 percent of patients classified as sporadic actually carried a disease-causing mutation in a known ALS gene. An additional 17 percent carried recognized genetic risk factors that increase susceptibility without directly causing the disease on their own. In total, roughly 28 percent of sporadic ALS patients had some identifiable genetic component. For the remaining 72 percent, no clear genetic clue was found with current testing methods.
This means the line between “genetic” and “non-genetic” ALS is blurrier than it once seemed. Many people with no family history still carry mutations that contributed to their disease.
The Major Gene Mutations
Four genes account for the majority of known genetic ALS cases: C9orf72, SOD1, FUS, and TARDBP. Each causes disease through different mechanisms, and knowing which one is involved matters for treatment.
C9orf72 is the most common culprit. It contains a short stretch of DNA that normally repeats a small number of times. In people with the mutation, that stretch repeats hundreds or even thousands of times. This abnormal expansion causes problems in multiple ways: the cell produces toxic clumps of RNA that get stuck in the nucleus, generates abnormal proteins that damage energy-producing structures in the cell and interfere with DNA repair, and loses the normal function of the C9orf72 protein itself. The C9orf72 mutation alone was found in about 5 percent of sporadic ALS patients in the Brain study.
SOD1 was the first ALS gene discovered and is the target of the only gene-specific ALS treatment currently available. Mutations in FUS and TARDBP are less common but tend to cause disease through the buildup of misfolded proteins inside motor neurons. A separate set of rare gene mutations, including changes in ALS2, SETX, and FUS, are associated with juvenile ALS, a form that strikes before age 25.
How ALS Is Inherited
Most familial ALS follows an autosomal dominant pattern, meaning you only need one copy of the mutated gene (from one parent) to be at risk. If a parent carries a dominant ALS mutation, each child has a 50 percent chance of inheriting it. Less commonly, ALS is inherited in a recessive pattern, where both parents must pass on a copy of the mutation for the disease to develop. In rare instances, the inheritance is X-linked.
Inheriting an ALS mutation does not guarantee you will develop the disease. Some people carry a known disease-causing mutation their entire lives and never show symptoms, a phenomenon called reduced penetrance. This is part of what makes ALS genetics so complicated for families: a parent can silently carry a mutation and pass it on without ever being sick themselves.
The Role of Environment and Epigenetics
ALS is widely considered a disease where both genes and environment contribute. Epigenetic changes, which are chemical modifications to DNA that alter how genes are turned on or off, accumulate throughout life in response to diet, smoking, toxin exposure, aging, and other factors. These modifications can affect ALS risk and may help explain why two people with the same mutation can have very different outcomes. One might develop symptoms at 40, while another never develops the disease at all.
Researchers have identified distinct epigenetic signatures in ALS patients, particularly changes in DNA methylation patterns. These findings suggest that for many people, ALS results from a combination of genetic vulnerability and environmental triggers rather than genetics alone.
Genetic Testing for ALS
Genetic testing is available for people diagnosed with ALS and for their biological relatives. The process typically starts with testing the person who has ALS to identify a specific mutation. If one is found, family members can then pursue predictive testing to learn whether they carry the same variant. Guidelines recommend that predictive testing happen alongside genetic counseling, given the emotional weight and potential insurance implications of the results.
Health insurance often covers testing when a doctor recommends it, though coverage varies by plan. For those without coverage, self-pay costs are typically lower than what labs charge insurers. A program called ALS Identified, sponsored by Biogen and offered through Invitae, provides free genetic testing to people with ALS and their families.
Why Genetic Testing Matters for Treatment
Knowing your genetic status has become more than academic. In 2023, the FDA approved Qalsody (tofersen), the first ALS treatment designed for a specific genetic subtype. It targets ALS caused by SOD1 mutations, working by reducing the production of the toxic SOD1 protein. In a clinical trial of 108 patients, those receiving the drug showed significant reductions in a blood marker of nerve damage compared to placebo, a result considered reasonably likely to predict clinical benefit.
Qalsody only works for people with SOD1 mutations, which underscores why genetic testing is increasingly viewed as a standard part of ALS care. As more gene-targeted therapies enter development, identifying the specific genetic cause, or ruling one out, will shape treatment options in a way that wasn’t possible even a few years ago.