ALS (amyotrophic lateral sclerosis) has no single cause. It results from a combination of genetic vulnerability, cellular malfunction, and environmental exposures that together destroy motor neurons, the nerve cells controlling voluntary movement. About 5% to 10% of cases run in families, while the remaining 90% to 95% appear without any known family history. Even in those sporadic cases, genetics, toxic protein buildup, and inflammatory damage all play overlapping roles.
Genetic Mutations Behind ALS
Several gene mutations are linked to ALS, and the most common one involves a gene called C9orf72. In healthy people, a short DNA sequence in this gene repeats a handful of times. In people with ALS-linked mutations, that sequence repeats hundreds or even thousands of times. This expansion causes damage in three ways: it reduces the gene’s normal output, it creates toxic clumps of RNA inside the cell nucleus, and it produces small, abnormal proteins that accumulate and poison the neuron. Those toxic proteins are especially harmful because they disrupt the cell’s ability to shuttle essential molecules between the nucleus and the rest of the cell, a transport system motor neurons depend on to survive.
Another well-studied gene, SOD1, was the first to be linked to ALS. Over 200 different mutations have been identified in this gene, and about 90% of them are single-letter changes that destabilize the protein it produces. The SOD1 protein normally helps neutralize damaging oxygen molecules inside cells. When mutated, the protein misfolds, loses a critical internal bond that holds its shape together, and begins clumping into aggregates. These aggregates are directly toxic to motor neurons.
More recently, researchers identified mutations in a gene called NEK1, which appear in nearly 3% of all ALS cases. Loss-of-function variants in NEK1 carry roughly an eightfold increased risk for familial ALS, and the association holds in sporadic cases as well. While less common individually, dozens of other genes have been connected to ALS risk, painting a picture of a disease with many possible genetic entry points.
Toxic Protein Buildup in Motor Neurons
Regardless of the genetic trigger, roughly 95% of all ALS cases share one molecular feature: clumps of a protein called TDP-43 accumulate in the cytoplasm of motor neurons. In healthy cells, TDP-43 stays in the nucleus, where it helps manage gene activity. In ALS, the protein gets displaced into the surrounding cell body, becomes chemically modified, and forms insoluble aggregates the cell cannot break down. The loss of TDP-43 from the nucleus disrupts normal gene regulation, while its toxic buildup in the cytoplasm damages cellular machinery. This dual hit, loss of function where it belongs and toxic gain of function where it doesn’t, is considered the central pathological event in nearly all non-SOD1 forms of ALS.
Glutamate and Nerve Cell Overstimulation
Motor neurons communicate using glutamate, the brain’s primary excitatory chemical signal. After a nerve impulse fires, specialized transporter proteins on surrounding support cells rapidly clear glutamate from the gap between neurons. In ALS, this cleanup system fails. Studies of ALS patients have found up to a 95% decrease in the key transporter protein (called EAAT2) in affected regions, even though the genetic instructions for making that protein remain normal. The problem appears to be that the protein gets degraded or pulled from the cell surface before it can do its job.
When glutamate lingers in the synapse, it overstimulates the receiving neuron. This floods the cell with calcium, which at high levels triggers a cascade of damage to mitochondria, DNA, and cell membranes. The dying neuron then releases even more glutamate, creating a vicious cycle of excitotoxic damage that spreads to neighboring motor neurons. This mechanism is one of the best-validated in ALS. Riluzole, one of the few approved treatments for the disease, works by dampening glutamate signaling.
Inflammation That Accelerates Damage
The brain and spinal cord have their own immune cells, called microglia, that normally patrol for damage and clear debris. In ALS, these cells shift into a chronically inflamed state. Rather than protecting neurons, they begin releasing inflammatory molecules and oxygen radicals that directly damage motor neurons. They also convert neighboring support cells called astrocytes into a toxic form, amplifying the destruction.
Research on ALS patients has found a fourfold increase in a key inflammatory signaling molecule (NF-κB) in the spinal cord. In animal models of ALS, blocking this inflammatory pathway in microglia extended survival by 20 days, a substantial effect that underscores how much of the disease’s progression is driven not by the initial genetic or molecular insult, but by the immune system’s destructive overreaction to it.
Environmental and Occupational Exposures
Not all ALS risk comes from inside the cell. Certain environmental exposures appear to increase the likelihood of developing the disease, particularly heavy metals. A prospective study that measured blood metal levels before disease onset found that people with the highest levels of cadmium had roughly double the ALS risk compared to those with the lowest levels. Lead showed a similar trend, with an approximately 89% increased risk in the highest exposure group. Zinc, interestingly, showed the opposite pattern, with higher levels linked to lower risk.
Military veterans have consistently shown higher rates of ALS than the general population, though the specific causal factor has not been identified. Potential contributors include exposure to toxins, intense physical exertion, and trauma, but the evidence remains too limited to pinpoint a single explanation.
Head Trauma and Contact Sports
One of the stronger environmental links involves repetitive head and neck injuries. A systematic review found that professional athletes in sports prone to concussive head and cervical spine trauma, such as American football and soccer, had an ALS risk roughly 8.5 times higher than the general population. That elevated risk was specific to professional-level contact sports. Nonprofessional athletes in the same sports showed no significant increase, and professional athletes in non-contact sports showed only a modest, statistically uncertain bump.
The combination of vigorous physical activity and repeated blunt trauma appears to be additive. Sports that involve both factors carry the highest risk, while either factor alone contributes much less. This pattern suggests that repeated injury to the brain and spinal cord, rather than exercise itself, is the primary driver. The mechanism likely involves chronic damage to motor neurons and their surrounding support structures in the cervical spinal cord and brainstem, regions that control the muscles most commonly affected early in ALS.
Why Most Cases Have No Clear Cause
For the majority of people diagnosed with ALS, no single gene mutation, toxic exposure, or injury can be identified as the trigger. The current understanding is that ALS develops when multiple risk factors converge: a genetic background that makes motor neurons slightly more vulnerable, combined with environmental stressors, aging-related decline in protein cleanup systems, and inflammatory changes that tip the balance. Each of the mechanisms described above, protein aggregation, glutamate toxicity, immune overactivation, and environmental insults, feeds into the others. TDP-43 buildup triggers inflammation, inflammation impairs glutamate clearance, excess glutamate kills neurons, and dying neurons release more toxic proteins. This interconnected cascade explains why the disease progresses relentlessly once it begins, and why a single cause has been so difficult to isolate in most patients.