For example, SOD1, the first gene found to be associated with ALS, has over 160 reported mutations associated with phentotypes that range from a rapidly progressing form of the disease to a much milder, slowly progressing form.26,27 In the case of the C9orf72 repeat expansion, healthy individuals typically have 10 or fewer repeats, and patients with ALS or FTD associated with this variant have between 30 and many thousands of repeats. Here, C9orf72 is linked to amyotrophic lateral sclerosis.