Most cases of amyotrophic lateral sclerosis (ALS) are sporadic. That means they occur due to changes in your genes (mutations) that occur during your lifetime.

However, in about 10% of cases, ALS develops in a person with a family history of the condition. Doctors define these cases as familial ALS, indicating that the genetic mutations may have been inherited — passed down from parent to child.

Still, inheriting a gene for ALS doesn’t mean you’ll develop the disease. And even if you do, symptoms don’t occur until later in life.

According to the ALS Association, scientists have identified more than 40 genes that may contribute to the development of ALS. However, four of these are responsible for more than 70% of ALS cases:

  • C9orf72
  • SOD1
  • FUS
  • TARDBP

Still, a mutation in any one of these genes doesn’t necessarily mean you’ll develop ALS. A gene’s likelihood to cause a disease is known as its penetrance.

Genes like C9orf72 and SOD1 have a very high penetrance, meaning you’re very likely to develop ALS. About half of people with the most common ALS-linked gene variant (C9orf72) develop symptoms by age 58. Almost all develop symptoms by age 80.

Other genes may only increase your ALS risk, but developing the disease may depend on other contributing factors.

Even if you’re born with the genes that contribute to ALS, symptoms don’t start until later in life. Symptoms typically begin between the ages of 55 to 75.

However, the onset of symptoms can vary depending on the genetic subtype. For example, the FUS gene variant causes an earlier age of onset than the SOD1 and C9orf72 gene variants.

Symptoms and complications also vary depending on the genes affected.

Rarely, ALS can develop in childhood. Doctors define juvenile ALS as any case that begins before age 25, but it can occur in early childhood.

Different genes may be responsible for juvenile ALS, including:

Doctors don’t screen for ALS as part of routine prenatal screening. However, genetic testing may be an option later in life for people with a strong family history of ALS.

Knowing whether you carry a specific genetic mutation may provide information as to your likeliness of developing ALS and even what complications you may experience or when symptoms may begin.

Still, it’s important to understand that ALS development depends on several factors. A genetic test will not provide you with a complete roadmap of your disease course. And even if you don’t carry a known genetic mutation, it’s still possible to develop ALS.