C9orf72 and frontotemporal dementia: A GGGGCC pathogenic expansion in the C9orf72 gene is the main genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS).1,2 Since its discovery, intriguing links appeared between this GGGGCC expansion and polyglutamine (polyQ) disorders due to exonic CAG expansions, leading to Huntington disease (HD) and several spinocerebellar ataxias.