The discovery that GGGGCC expansions in the C9orf72 gene are the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), collectively termed C9FTD/ALS, has transformed our understanding of these diseases (Rohrer et al, 2015). This evidence concerns the gene C9orf72 and frontotemporal dementia.