C9orf72 and amyotrophic lateral sclerosis: The majority of ALS patients have the sporadic form of the disease (sALS), but about 10% of the cases have familial ALS (fALS), which is associated with pathogenic mutations in genes such as superoxide dismutase 1 (SOD1), transactive response DNA-binding protein 43 (TARDBP encoding TDP43), and C9ORF72, which is characterized by an intronic hexanucleotide expansion.