A GGGGCC hexanucleotide repeat expansion (HRE) in C9orf72 is the most common known cause of amyotrophic lateral sclerosis (ALS) and is also a major cause of frontotemporal dementia (FTD) and the ALS/FTD overlap syndrome (DeJesus-Hernandez et al., 2011; Renton et al., 2011; Majounie et al., 2012). The gene discussed is C9orf72; the disease is frontotemporal dementia.