C9orf72 and frontotemporal dementia: A massive expansion of the GGGGCC hexanucleotide repeat in the intron between non-coding exons 1a and 1b of the chromosome 9 open reading frame 72 (C9orf72) gene was recently found to be a major genetic cause of familial frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) (1, 2).