While most ALS cases have no known genetic cause, an intronic GGGGCC hexanucleotide repeat expansion (HRE) in the C9ORF72 gene is responsible for the majority of familial ALS (fALS) and a subset of sporadic ALS (sALS) [17, 67]. The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.