C9orf72 and frontotemporal dementia: Finally, several studies have shown that the pathogenicity related to the GGGGCC hexanucleotide-repeat-expansion within the first intron of the C9ORF72 gene, which is the most frequent mutation found in patients of ALS and also a major cause of frontotemporal dementia (FTD) and the ALS/FTD overlap syndrome [149,231,232], might be related to the production of arginine-rich cell-penetrating peptides (CPPs).