About 10 % of all patients with amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) or mixed presentation of both diseases (ALS/FTD) are caused by a massive expansion of a GGGGCC repeat upstream of the C9orf72-coding region [11, 18, 43]. The gene discussed is C9orf72; the disease is frontotemporal dementia.