About 5% to 10% of ALS cases are hereditary, meaning they run in families through an identified genetic link. The remaining 90% to 95% of cases are considered sporadic, appearing in people with no known family history of the disease. But the line between hereditary and sporadic ALS is blurrier than those numbers suggest, because genetic mutations play a role in some sporadic cases too.
Familial vs. Sporadic ALS
ALS is classified as “familial” when a person has one or more blood relatives who also had the disease. A large meta-analysis across 165 studies found that familial cases make up roughly 8% of all ALS diagnoses, with population-based studies settling closer to 5%. The exact number depends on how thoroughly a family’s medical history has been documented. Some families may not realize ALS has appeared before, especially if older relatives were misdiagnosed or passed away before a clear diagnosis was made.
Sporadic ALS, the more common form, occurs without any obvious family pattern. For a long time, researchers assumed sporadic cases were driven entirely by environmental factors or random biological events. That picture has changed. Around 10% of people with sporadic ALS carry a known disease-associated genetic mutation, suggesting that genetics can contribute even when there’s no visible family history. A mutation can appear for the first time in one person (a “de novo” mutation), or it can sit quietly in a family for generations without causing symptoms due to incomplete penetrance.
How ALS Is Inherited
Most familial ALS follows an autosomal dominant inheritance pattern. This means only one copy of the mutated gene, inherited from one parent, is enough to cause the disease. If a parent carries a dominant ALS mutation, each of their children has a 50% chance of inheriting that same mutation.
A smaller number of ALS-linked genes follow autosomal recessive inheritance, which requires a mutated copy from both parents. A few rare forms are X-linked, meaning the gene sits on the X chromosome and affects males and females differently. But the vast majority of hereditary ALS families see a dominant pattern, where the disease can appear in every generation.
The Major Genes Involved
More than 30 genes have been linked to ALS, but four account for the largest share of familial cases:
- C9orf72: The most common genetic cause, responsible for over 30% of familial ALS cases. This mutation involves a short stretch of DNA that repeats far more times than it should, disrupting normal cell function. It is also linked to frontotemporal dementia (FTD), a condition that affects behavior, personality, and language. Some carriers develop ALS, some develop FTD, and some develop both.
- SOD1: The first ALS gene ever discovered, accounting for about 20% of familial cases. SOD1 mutations cause a toxic buildup of a misfolded protein that damages motor neurons. Different SOD1 mutations can produce very different outcomes. One particular variant is associated with rapid progression, with survival often under 18 months after diagnosis.
- TARDBP and FUS: Together these account for roughly 4% to 5% of familial cases. Both genes are involved in how cells process RNA, a molecular cousin of DNA that helps build proteins.
The remaining familial cases are spread across dozens of rarer genes, and some families with clear inheritance patterns still have no identified mutation, meaning undiscovered genes are likely involved.
Carrying a Gene Doesn’t Guarantee Disease
One of the most important and least intuitive facts about hereditary ALS is that carrying a mutation does not mean you will develop symptoms. This concept, called penetrance, describes how likely a gene carrier is to actually get the disease.
C9orf72 illustrates this well. Early studies of large affected families suggested the mutation had near-complete penetrance by age 80, meaning almost every carrier eventually developed ALS or FTD. But when researchers looked at the broader population, the picture changed dramatically. Population-level estimates put the lifetime penetrance of C9orf72 at roughly 33%, with ALS-specific risk estimates ranging from about 24% to 44%. In other words, many carriers live full lives without ever developing symptoms.
Penetrance also varies by age and sex. Risk increases as carriers get older, and some studies suggest differences between men and women in when symptoms are most likely to appear. This incomplete penetrance is one reason ALS can seem to skip generations in a family, making it harder to recognize a hereditary pattern.
Genetic Testing for ALS
Consensus guidelines now recommend that all people diagnosed with ALS be offered genetic testing, regardless of whether they have a family history. The standard panel tests for the four most common genes: C9orf72, SOD1, FUS, and TARDBP. Testing has become more relevant in recent years because a treatment now exists for one specific genetic form of the disease.
In 2023, the FDA approved a therapy specifically for ALS caused by SOD1 mutations. The drug works by reducing production of the toxic SOD1 protein. It was approved under an accelerated pathway based on its ability to lower a blood marker of nerve damage. This makes SOD1 testing directly actionable: if you carry the mutation, a targeted treatment is available. Researchers are working on similar gene-targeted approaches for C9orf72 and other mutations.
For people already diagnosed, identifying a genetic cause can clarify prognosis and open doors to clinical trials designed for specific mutations. Some SOD1 variants are associated with earlier onset, while others progress more slowly. Knowing your specific mutation can help your care team anticipate what to expect.
Testing for Family Members
When a genetic mutation is found in someone with ALS, their biological relatives face a difficult decision: whether to get tested themselves. This is called predictive testing, and it carries significant emotional weight. A positive result means living with the knowledge that you carry a mutation linked to a serious disease, even though you may never develop symptoms.
Genetic counseling before and after testing is strongly encouraged. Counselors help family members understand what a result does and does not mean, explore how it might affect their emotional wellbeing and family relationships, and discuss reproductive options for those planning to have children. The conversation often extends beyond the individual being tested, since one person’s result reveals information about siblings, parents, and children as well.
Timing is a deeply personal consideration. Some families choose to wait until treatments become available for their specific mutation, reasoning that there is limited benefit to knowing your status if nothing can be done. Others want the information immediately for family planning or to enroll in prevention trials. There is no universally right answer, and different family members often make different choices. Some research participants have described complex family dynamics around testing, with parents holding off on sharing genetic information or relatives disagreeing about whether and when to pursue results.