C9orf72 and frontotemporal dementia: The GGGGCC hexanucleotide repeat expansion (HRE) in the first intron of C9orf72 (MIM: 614260) is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (FTDALS1 [MIM: 105550]) in populations of European descent.1