FTD-C9orf72 is the most common genetic form of FTD in North America and Europe, responsible for approximately 13% of all FTD cases and approximately 11% of all ALS (ALS-C9orf72) cases; it can also cause a combination of ALS and FTD.58 FTD-C9orf72 results from a hexanucleotide repeat expansion within the C9orf72 gene, and there is compelling evidence to support several different mechanisms that cause this repeat to result in TDP43 neuropathology and degeneration. This evidence concerns the gene C9orf72 and amyotrophic lateral sclerosis.