The most commonly identified genetic cause of ALS and FTD involves polymorphic repeat expansions, composed of hundreds to thousands of the GGGGCC hexanucleotide-repeat sequence (hereafter abbreviated G4C2) in the first intron of the C9ORF72 gene, with autosomal dominant inheritance and incomplete penetrance1, 2, 3, 4. This evidence concerns the gene C9orf72 and amyotrophic lateral sclerosis.