It's worth keeping in mind that most common neurodegenerative diseases are syndromes: clusters of diseases defined by symptoms and certain aspects of the disease process rather than by root cause. Even Huntington disease is actually a class of genetic mutation rather than one specific mutation: a sequence of three nucleotides is repeated many times, but the number of repetitions varies and affects the severity and age of onset. There are dozens of different mutations known to cause ALS, and severity, age of onset, and specific symptoms vary accordingly.