Since the discovery that a hexanucleotide repeat expansion (HRE) composed of the GGGGCC (G4C2) sequence within the first intron of the C9ORF72 gene is the most common cause of C9ALS/FTD [8, 9], major efforts have been undertaken to comprehend the underlying mechanism of this mutation (reviewed in [7, 10–14]). Here, C9orf72 is linked to frontotemporal dementia.