Two types of ALS have different causes

Amyotrophic laterals sclerosis (ALS, or Lou Gehrig's disease) is either inherited (about 5 percent of cases) or occurs in a person with no family history of the disease (sporadic form). It has been assumed that these two forms of ALS have a similar cause. A study conducted by researchers at the University of Pennsylvania School of Medicine found that's not the case. The inherited form is associated with abnormal SOD-1 proteins, but the sporadic form is associated with abnormal TDP-43 proteins. The researchers point out that if drug treatments and research studies only focus on SOD-1 proteins this will not benefit people with the sporadic form of ALS. The research was published online April 27, 2007, in Annals of Neurology.

Source: Mackenzie, Ian, et al. "Pathological TDP-43 distinguishes sporadic amyotrophic lateral sclerosis from amyotrophic lateral sclerosis with SOD1 mutations." Annals of Neurology 68(2007): 427-434.