C9orf72 and frontotemporal dementia: 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.