C9orf72 and amyotrophic lateral sclerosis: ALS is classified into sporadic (90–95%) and familial (FALS, 5–10%).5 At least 30 genes have been found to cause FALS, of which the most common is the intronic hexanucleotide expansion within the C9orf72 gene which is present in ∼40% of FALS cases and ∼7% of ALS cases with apparently sporadic disease.