C9orf72 and frontotemporal dementia: 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.