Expansions in a hexanucleotide repeat (GGGGCC) in a noncoding region of the C9orf72 gene are the most common cause of familial and sporadic ALS and FTD to date, accounting for roughly 40% of familial ALS and 5–10% sporadic ALS [34, 128]. The gene discussed is C9orf72; the disease is frontotemporal dementia.