In 2011, abnormal expansion of a GGGGCC hexanucleotide repeat in a predicted non-coding region of the chromosome 9 open reading frame 72 (C9orf72) gene was identified as the most common genetic cause of familial and sporadic forms of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) including families in which both conditions co-occur [13, 40]. The gene discussed is C9orf72; the disease is frontotemporal dementia.