C9orf72 and amyotrophic lateral sclerosis: At the C9orf72 locus, a hexanucleotide repeat expansion with 100% GC content (GGGGCC) can expand into the hundreds, and even thousands, of repeats, and is the most common heritable cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) [5], [6], [7].