C9orf72 and frontotemporal dementia: The C9ORF72 locus has been researched extensively following identification of an expanded hexanucleotide (GGGGCC) repeat in C9ORF72 as the most common cause of sporadic and familial forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD)1, 2.