C9orf72 and frontotemporal dementia: Genome-wide association studies of ALS led to the identification of a hexanucleotide repeat expansion (HRE) in C9ORF72 as a major genetic cause of ALS/FTD, and 15 genes are robustly associated with ALS in a cross-ancestry manner7, 8, some of them having been found causative in familial ALS/FTD cases (eg C9ORF72, SOD1, KIF5A or TBK1).