A hexanucleotide repeat expansion in C9orf72 is common in familial amyotrophic lateral sclerosis (ALS) with frontotemporal dementia (FTD) (C9-ALS/FTD) (DeJesus-Hernandez et al., 2011), which leads to the production of proline:arginine (PR), glycine:arginine (GR), glycine:alanine (GA), proline:alanine (PA), and glycine:proline (GP) poly-dipeptides (Ash et al., 2013; Kwon et al., 2014). The gene discussed is C9orf72; the disease is frontotemporal dementia.